rbenv-plugin should always release the lock

https://issues.jenkins-ci.org/browse/JENKINS-24002

I have found a few situations where rbenv-plugin will try to release a the .rbenv.lock and the file was already removed. In that situation the script will fail. I believe the rbenv-plugin will never be in a situation where it does not want to remove the lock file and should continue if the file is gone for some other reason.