Ensure chruby-exec parameters are shell-quoted.

- Preserve quotes in chruby-exec command parameters (closes #283).
- Fixes a bug where RUBYOPT options passed to chruby-exec are ignored.
Merge pull request #285 from havenwood/chruby-exec-parameters

Ensure chruby-exec parameters are shell-quoted.
Match comments in setup.sh to what the script actually does.
Recommend the chruby_install chef cookbook instead (closes #287).
Enable OSX testing on Travis CI.
Added a requirements section.