bundle installでRMagickをインストール中にエラーが発生したのでそれの解決法についてメモっておきます．
以下がbundle install中のエラーメッセージ．
Installing rmagick (2.13.1) with native extensions
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
/Users/ユーザ名/.rbenv/versions/1.9.3-p194/bin/ruby extconf.rb
checking for Ruby version >= 1.8.5... yes
extconf.rb:128: Use RbConfig instead of obsolete and deprecated Config.
checking for gcc... yes
checking for Magick-config... yes
checking for ImageMagick version >= 6.4.9... yes
checking for HDRI disabled version of ImageMagick... yes
Package MagickCore was not found in the pkg-config search path.
Perhaps you should add the directory containing `MagickCore.pc'
to the PKG_CONFIG_PATH environment variable
No package 'MagickCore' found
Package MagickCore was not found in the pkg-config search path.
Perhaps you should add the directory containing `MagickCore.pc'
to the PKG_CONFIG_PATH environment variable
No package 'MagickCore' found
Package MagickCore was not found in the pkg-config search path.
Perhaps you should add the directory containing `MagickCore.pc'
to the PKG_CONFIG_PATH environment variable
No package 'MagickCore' found
Package MagickCore was not found in the pkg-config search path.
Perhaps you should add the directory containing `MagickCore.pc'
to the PKG_CONFIG_PATH environment variable
No package 'MagickCore' found
checking for stdint.h... yes
checking for sys/types.h... yes
checking for wand/MagickWand.h... no
Can't install RMagick 2.13.1. Can't find MagickWand.h.
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
Provided configuration options:
--with-opt-dir
--without-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/Users/ユーザ名/.rbenv/versions/1.9.3-p194/bin/ruby
Gem files will remain installed in /somewhere/vendor/bundle/ruby/1.9.1/gems/rmagick-2.13.1 for inspection.
Results logged to /somewhere/vendor/bundle/ruby/1.9.1/gems/rmagick-2.13.1/ext/RMagick/gem_make.out
An error occured while installing rmagick (2.13.1), and Bundler cannot continue.
Make sure that `gem install rmagick -v '2.13.1'` succeeds before bundling.
色々書かれていますが，要するにwand/MagickWand.hが見つからないっていうことだと判断．ImageMagick自体はインストールされているはずなので，locate wand/MagickWand.hで該当ファイルがあるかどうかを確認．
/usr/local/include/ImageMagick/wand/MagickWand.h
/usr/local/src/ImageMagick-6.7.4-4/wand/MagickWand.h
ものはあるらしい，ということでこの場所を参照していないんだろうと推測．
と，ここまで来てエラーメッセージを見返すと
Perhaps you should add the directory containing `MagickCore.pc'
to the PKG_CONFIG_PATH environment variable
とのこと．なんだ書いてあるじゃないか！ということでlocate MagickCore.pcをするとどうやら/usr/local/lib/pkgconfig/にあるらしい．
更にenv | grep PKG_CONFIG_PATHをするとPKG_CONFIG_PATHに何も設定されていないことがわかったので，export PKG_CONFIG_PATH=/usr/local/lib/pkgconfigで設定．
これで改めてbundle installをしてみたらちゃんと入ってくれました．