Prevent JRuby rubygems warnings from breaking the build

/cc @myronmarston 
Fair enough.

Semantically, I still think we shouldn't be setting an expectation about this warning.  Also, I'm guessing that you've defined your regex will fail if there is extra stderr output, but I haven't been able to convince myself of that by reading the regex.  (I think your regex-fu is stronger than mine).  One can also imagine a subtle change to the regex breaking that and allowing additional warnings without us noticing.  Something like this seems simpler, more robust and more maintainable:

``` ruby
non_rubygems_lines = stderr.lines.reject { |line| line.include?("lib/ruby/shared/rubygems/defaults/jruby") }
expect(non_rubygems_lines).to eq([])
```

All that said, this doesn't affect end users and I want this merged soon so I'll defer to you on this if you prefer it how it is.