bump jruby to 1.7.16 and add dev build

So, `jruby:dev` is something that is going to make sense and be intuitive for users of this image?  (as opposed to `jruby:tip`, `jruby:master`, `jruby:head`, `jruby:9000`, or any other machination)

Just making sure since I'm not a JRuby user, so I don't know what they'd expect or understand this to mean. :smile: