RVM Support

[ysbaddaden]

The rvm branch and this pull request will eventually make Prax correctly handle RVM.

The first commit removes any specific Ruby from $PATH which allows Prax to run Racker from a clean shell which should let RVM do it's job of choosing the correct ruby and gemset for the project.

This looks like to be enough with a simple Rack application (setting .ruby-version and restarting the instance actually selects the correct Ruby version):

run(lambda do |env|
  [200, {}, [RUBY_VERSION, "\n"]]
end)

[ysbaddaden]

There still is the following warning, but it looks like this is unavoidable (bin/prax prepends the libexec directory to $PATH) before running libexec/prax-start:

Warning! PATH is not properly set up, '/usr/local/rvm/gems/ruby-2.1.5/bin' is not at first place,
         usually this is caused by shell initialization files - check them for 'PATH=...' entries,
         it might also help to re-add RVM to your dotfiles: 'rvm get stable --auto-dotfiles',
         to fix temporarily in this shell session run: 'rvm use ruby-2.1.5'.

[ysbaddaden]

@mpapis is it possible to avoid the warning message? If not, then I'm ok.

[mpapis]

the warning is displayed when you have GEM_HOME / GEM_PATH and they are not in sync with PATH - if you clean PATH then also clean the two other variables