rbenv:validate is now a 'success' if a Ruby version exists.

Also, 'exit 1' is not needed as the correct exit value is set by the
[ -d $dir ] query.
Reverted removal of 'exit 1' line.
Changelog updated, 2.0.3 entry added.
Merge pull request #36 from neektza/master

rbenv:validate is now a 'success' if version exists.