Change `exec` semantics to allow using rbenv from a ruby process

The most common pitfall with rbenv is that you can't shell out to
another rbenv version from a ruby process started with rbenv. #121

This is an unfortunate side-effect of a PATH-modifying hack to allow
support for `ruby -S`. #14

Here I propose that we stop modifying PATH during `rbenv-exec` unless
we're actually dealing with `ruby -S`.

The drawback of this change is that you still can't shell out to another
rbenv version from a process started with `ruby -S`. An alternative
solution to this change would be to re-implement the `ruby -S` semantics
and execute it without ever needing to change PATH.