set the default env prepending ~/.rbenv/shims and ~/.rbenv/bin to $PATH

Please see [stackoverflow this thread](http://stackoverflow.com/questions/19716131/usr-bin-env-ruby-no-such-file-or-directory-using-capistrano-3-capistrano-rben).

Using this plugin without setting the `default_env` was causing `/usr/bin/env ruby : Not such file or directory`. I figured how to solve it (see the solution I found in stackoverflow) and I this change fixes it.

Thank you!