Make sure to unset RUBY_AUTO_VERSION

Commit 1204c0642675d82f6132c06f17aa899cb6ecf5e5 made modifications which
renamed RUBY_VERSION_FILE to RUBY_AUTO_VERSION. This make sure we unset
the proper variable.
Merge pull request #181 from KevinSjoberg/master

Make sure to unset RUBY_AUTO_VERSION