Function to run command under multiple rubies

Using chruby, natch.

    $ chrubies 1.9,2.0,2.1 gem install faker
    >> 1.9 => ruby 1.9.3p484 (2013-11-22 revision 43786) [x86_64-darwin13.0.0]
    >> gem install faker:
    Successfully installed faker-1.2.0
    1 gem installed

    >> 2.0 => ruby 2.0.0p353 (2013-11-22 revision 43784) [x86_64-darwin13.0.0]
    >> gem install faker:
    Successfully installed faker-1.2.0
    Parsing documentation for faker-1.2.0
    1 gem installed

    chruby: unknown Ruby: 2.1