Here is a good example of how `chruby-exec` just doesn't load `chruby` correctly on OS X:

```
Last login: Sat Mar 22 08:27:50 2014 from 172.28.3.119
manhattan-slave-12006:~ build$ ruby --version
ruby 2.0.0p247 (2013-06-27 revision 41674) [universal.x86_64-darwin13]
manhattan-slave-12006:~ build$ gem list

*** LOCAL GEMS ***

CFPropertyList (2.2.0)
libxml-ruby (2.6.0)
nokogiri (1.5.6)
sqlite3 (1.3.7)
manhattan-slave-12006:~ build$ chruby --version
-bash: chruby: command not found
manhattan-slave-12006:~ build$ chruby-exec 2 -- gem list
bash: chruby: command not found
manhattan-slave-12006:~ build$ source /usr/local/share/chruby/chruby.sh
manhattan-slave-12006:~ build$ chruby --version
chruby: 0.3.8
manhattan-slave-12006:~ build$ chruby-exec 2 -- gem list
bash: chruby: command not found
manhattan-slave-12006:~ build$ chruby 2
manhattan-slave-12006:~ build$ ruby --version
ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-darwin13.0]
manhattan-slave-12006:~ build$ gem list

*** LOCAL GEMS ***

bigdecimal (1.2.4)
bundler (1.5.3)
io-console (0.4.2)
json (1.8.1)
minitest (4.7.5)
psych (2.0.3)
rake (10.1.0)
rdoc (4.1.0)
test-unit (2.1.1.0)
```