MacOSX 10.9
Xcode 5.1.1　ー＞ 6.2

１．Xcode を6.2に更新

Downloads for Apple Developers
https://developer.apple.com/download/more/

２．brew doctor でウォーニング

...
$ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry and just ignore them. Thanks!

Warning: You are using macOS 10.9.
We (and Apple) do not provide support for this old version.
You may encounter build failures or other breakages.
Please create pull-requests instead of filing issues.

Warning: Some keg-only formula are linked into the Cellar.
Linking a keg-only formula, such as gettext, into the cellar with
brew link <formula> will cause other formulae to detect them during
the ./configure step. This may cause problems when compiling those
other formulae.

Binaries provided by keg-only formulae may override system binaries
with other strange results.

You may wish to brew unlink these brews:
imagemagick@6
libxml2
...

３．「imagemagick@6」の削除とアップグレード

Homebrewのアップデート時に出た5つのWarningと解決方法 - Qiita
http://qiita.com/unsoluble_sugar/items/acaffa6d0e28c3c24934

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$ brew unlink imagemagick@6
$ brew install imagemagick@6
Error: imagemagick@6 6.9.8-0 is already installed
To upgrade to 6.9.9-8, run brew upgrade imagemagick@6
$ brew upgrade imagemagick@6

（中略）

/usr/local/Cellar/freetype/2.8: 63 files, 2.7MB, built in 23 seconds
==> Installing imagemagick@6
==> Downloading https://dl.bintray.com/homebrew/mirror/imagemagick%406-6.9.9-8.tar.xz
######################################################################## 100.0%
==> ./configure --disable-osx-universal-binary --prefix=/usr/local/Cellar/imagemagick@6/6.9.9-8 --disable-
==> make install
==> Caveats
This formula is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.

If you need to have this software first in your PATH run:
echo 'export PATH="/usr/local/opt/imagemagick@6/bin:$PATH"' >> ~/.bash_profile

For compilers to find this software you may need to set:
LDFLAGS: -L/usr/local/opt/imagemagick@6/lib
CPPFLAGS: -I/usr/local/opt/imagemagick@6/include
For pkg-config to find this software you may need to set:
PKG_CONFIG_PATH: /usr/local/opt/imagemagick@6/lib/pkgconfig

==> Summary
/usr/local/Cellar/imagemagick@6/6.9.9-8: 1,469 files, 23.3MB, built in 2 minutes 45 seconds
$
...

４．「libxml2」の削除とアップグレード

...
$ brew unlink libxml2
Unlinking /usr/local/Cellar/libxml2/2.9.4_2... 17 symlinks removed
$ brew install libxml2
Error: libxml2 2.9.4_2 is already installed
To upgrade to 2.9.4, run brew upgrade libxml2
$ brew upgrade libxml2
...

以上