Remove therubyracer runtime dependency

I don't think this has any runtime usage, I can remove it and v8* here with no apparent consequences.  It remains a build time dependency of course, and is required by foreman-assets still, enabling its use for asset precompilation.  I don't think there's any other use of a JavaScript engine.

http://koji.katello.org/koji/taskinfo?taskID=188942 f19
http://koji.katello.org/koji/taskinfo?taskID=188944 el6
http://koji.katello.org/koji/taskinfo?taskID=188948 el7

Just putting this up to check I'm not going mad.