$ brew update
/usr/local/bin/brew: /usr/local/Library/brew.rb: /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby: bad interpreter: No such file or directory
/usr/local/bin/brew: line 23: /usr/local/Library/brew.rb: Undefined error: 0

と言われたので、再インストールすることにした。
ちなみに、

$ ruby -v
ruby 2.0.0p195 (2013-05-14 revision 40734) [x86_64-darwin12.3.0]

http://brew.sh/index_ja.html
を参考に、

$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
==> This script will install:
/usr/local/bin/brew
/usr/local/share/doc/homebrew
/usr/local/share/man/man1/brew.1
/usr/local/share/zsh/site-functions/_brew
/usr/local/etc/bash_completion.d/brew
/usr/local/Homebrew
==> The following existing directories will be made group writable:
/usr/local/share/man/man8
==> The following existing directories will have their owner set to estis:
/usr/local/share/man/man8
==> The following new directories will be created:
/usr/local/Homebrew
/usr/local/Frameworks
/usr/local/sbin
/usr/local/share/zsh
/usr/local/share/zsh/site-functions
/usr/local/var

Press RETURN to continue or any other key to abort
==> /usr/bin/sudo /bin/chmod g+rwx /usr/local/share/man/man8
Password:
==> /usr/bin/sudo /usr/sbin/chown estis /usr/local/share/man/man8
==> /usr/bin/sudo /bin/mkdir -p /usr/local/Homebrew /usr/local/Frameworks /usr/local/sbin /usr/local/share/zsh /usr/local/share/zsh/site-functions /usr/local/var
==> /usr/bin/sudo /bin/chmod g+rwx /usr/local/Homebrew /usr/local/Frameworks /usr/local/sbin /usr/local/share/zsh /usr/local/share/zsh/site-functions /usr/local/var
==> /usr/bin/sudo /bin/chmod u+rwx share/zsh share/zsh/site-functions
==> /usr/bin/sudo /usr/sbin/chown estis /usr/local/Homebrew /usr/local/Frameworks /usr/local/sbin /usr/local/share/zsh /usr/local/share/zsh/site-functions /usr/local/var
==> /usr/bin/sudo /usr/bin/chgrp admin /usr/local/Homebrew /usr/local/Frameworks /usr/local/sbin /usr/local/share/zsh /usr/local/share/zsh/site-functions /usr/local/var
==> /usr/bin/sudo /bin/mkdir -p /Users/estis/Library/Caches/Homebrew
==> /usr/bin/sudo /bin/chmod g+rwx /Users/estis/Library/Caches/Homebrew
==> /usr/bin/sudo /usr/sbin/chown estis /Users/estis/Library/Caches/Homebrew
==> Downloading and installing Homebrew...
remote: Counting objects: 3534, done.
remote: Compressing objects: 100% (2481/2481), done.
remote: Total 3534 (delta 1664), reused 2256 (delta 914), pack-reused 0
Receiving objects: 100% (3534/3534), 2.23 MiB | 806.00 KiB/s, done.
Resolving deltas: 100% (1664/1664), done.
From https://github.com/Homebrew/brew
 * [new branch]      master     -> origin/master
 * [new tag]         0.1        -> 0.1
 * [new tag]         0.2        -> 0.2
 * [new tag]         0.3        -> 0.3
 * [new tag]         0.4        -> 0.4
 * [new tag]         0.5        -> 0.5
 * [new tag]         0.6        -> 0.6
 * [new tag]         0.7        -> 0.7
 * [new tag]         0.7.1      -> 0.7.1
 * [new tag]         0.8        -> 0.8
 * [new tag]         0.8.1      -> 0.8.1
 * [new tag]         0.9        -> 0.9
 * [new tag]         0.9.1      -> 0.9.1
 * [new tag]         0.9.2      -> 0.9.2
 * [new tag]         0.9.3      -> 0.9.3
 * [new tag]         0.9.4      -> 0.9.4
 * [new tag]         0.9.5      -> 0.9.5
 * [new tag]         0.9.8      -> 0.9.8
 * [new tag]         0.9.9      -> 0.9.9
 * [new tag]         1.0.0      -> 1.0.0
 * [new tag]         1.0.1      -> 1.0.1
 * [new tag]         1.0.2      -> 1.0.2
 * [new tag]         1.0.3      -> 1.0.3
 * [new tag]         1.0.4      -> 1.0.4
 * [new tag]         1.0.5      -> 1.0.5
 * [new tag]         1.0.6      -> 1.0.6
 * [new tag]         1.0.7      -> 1.0.7
 * [new tag]         1.0.8      -> 1.0.8
HEAD is now at 5b421b9 Merge pull request #1329 from jawshooah/cask/cleanup-tests
==> Homebrew has enabled anonymous aggregate user behaviour analytics
Read the analytics documentation (and how to opt-out) here:
  https://git.io/brew-analytics
==> Tapping homebrew/core
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core'...
remote: Counting objects: 3740, done.
remote: Compressing objects: 100% (3630/3630), done.
remote: Total 3740 (delta 12), reused 301 (delta 1), pack-reused 0
Receiving objects: 100% (3740/3740), 2.99 MiB | 3.24 MiB/s, done.
Resolving deltas: 100% (12/12), done.
Checking connectivity... done.
Tapped 3619 formulae (3,767 files, 9.3M)
Already up-to-date.
Error: Could not link:
/usr/local/share/man/man1/brew.1

Please delete these paths and run `brew update`.
==> Installation successful!
==> Next steps
Run `brew help` to get started
Further documentation: https://git.io/brew-docs

Error: Could not link:
/usr/local/share/man/man1/brew.1

って言われたが、

Installation successful!
らしい。

$ brew update
Already up-to-date.

最新版になった。