rbenv exec: avoid mutating PATH

Enables shelling out from a ruby process started with rbenv to a ruby
process with a different RBENV_VERSION. Fixes #121

This removes the workaround created for #15 and solves `ruby -S` support
by setting RUBYPATH. PATH is never changed.

To illustrate how RUBYPATH changes in various configurations:

    PATH=~/bin:~/.rbenv/shims:/usr/bin:/bin
    RBENV_VERSION=1.8 ruby -S rake
    #=> executes ~/.rbenv/versions/1.8/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/1.8/bin:/usr/bin:/bin

    RBENV_VERSION=2.0 ruby -S rake
    #=> executes ~/.rbenv/versions/2.0/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/2.0/bin:/usr/bin:/bin

    RBENV_VERSION=system ruby -S rake
    #=> executes /usr/bin/rake
    #=> RUBYPATH=~/bin:/rbenv_shims_were_here:/usr/bin:/bin

    RBENV_VERSION=1.8 ruby -S rake
    #=> executes ~/.rbenv/versions/1.8/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/1.8/bin:/usr/bin:/bin