Allow user to override the ruby used when running a one-off-job

user's can put a .ruby-version file in their projects now and chruby will auto-switch to it. This is nice, since it keeps the ruby-version in the project it's specific to, and since it doesn't require people to sprinkle "chruby XXX" around their scripts - one day, someone will come up with something even better than chruby, but as long as it still honors .ruby-version files, our scripts won't care what the ruby manager is.