Ruby version updates shouldn't touch the file

[chrismo]

A ruby app with .ruby-version 2.3.1, run bundle patch -v -r --rubies 2.3.1 - no content changes, BUT the timestamp on the .ruby-version file is touched. That will cause the git library to detect that change, even though CLI won't see it.

Minor, minor issue - but came across it inside LivingSocial