Fix test runs for optional dependencies

I've tidied up the way we require dev dependencies, so that the same dependencies are loaded in all test runs on travis (previously the test runs using any of the Gemfiles in `./gemfiles/` were not loading minitest 5.

Unfortunately, I think that (because of this) since we upgraded to minitest, the builds for optional dependencies haven't been running any tests at all - looking at the build logs, the builds for yajl, multi_json etc have been finding 0 test files (and passing without doing any work).
Actually, this is a mistake - we don't want to do this. The underlying problem is actually in the data the common test suite is using...