Removed support for specific version managers

This is an attempt at skipping support for specific version managers as per #36, while still permitting the user to choose the ruby version under which Racker must run.

It works by changing the call to Process.spawn to a string instead of a list of argument, which forces the command to be run through a shell and not directly from the ruby process. We also clean the environment from everything except for PATH which is set to ORIG_PATH, which is the PATH before Prax is started, thus skipping any ruby version changes.

It works perfectly with **rbenv**. Please tell me if it works with **RVM** and **chruby**!