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

users can now set a RUBY_FOR_USER_SCRIPT environment variable. Otherwise, it will default to the first ruby it finds in the /opt/rubies folder.
Update chruby to automatically switch to the .ruby-version it finds on cd'ing into a directory
Forwards traps to shell functions and source the PIPELINE_USER_SCRIPT

Otherwise, chruby auto.sh functionality won't carry forward into scripts you are executing. This does assume that your pipeline script is a `source`able shell script.
Merge pull request #2 from pivotal-tempest/patch-1

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