Fix bundled git gems

This tweaks the behavior of `redstorm bundle` slightly to make it a bit more resilient to two main cases: (1) where the project uses gems sourced from git (fixing #51) and (2) where the project is itself a gem.

For the former, instead of copying the gemspec (which generally uses `git ls-files` to generate the list of files), render the gemspec object back to Ruby and save that.

For the latter, skip over any gem that contains the `target/gems/` directory; otherwise the gem copy will recurse infinitely.