`JRubyEnvironment#onStart` sets the current context classloader.

It turns out this is part of the standard jruby boot process (when
started via its `Main`) and necessary to prevent an obscure HBase
classloader [bug](http://stackoverflow.com/a/18599300/580412).

This requires us to parse the jruby argv earlier in the process, and
unfortunately I can't seem to make `RubyMain` work with it for some
reason (it just ignores the inline script?) so I ditched it.