Enable coveralls + simplecov coverage reports

This updates every test to just require a common `test_helper`, which boots SimpleCov with two reporters: one to spit out a local report in `coverage/index.html`, and another to submit to [Coveralls](http://coveralls.io).

I sent this PR to my own fork first so we could get an example of the result:

1. https://coveralls.io/r/pd/json-schema -- repo page showing latest builds and current status
2. https://coveralls.io/builds/1389205 -- actual build report

@iainbeeston [mentioned](https://github.com/hoxworth/json-schema/pull/141#issuecomment-60557532) getting the report coverage into codeclimate, which might be a better choice since we're already using it. I mostly just wanted a POC to validate that we could get this report generating at all.

The only benefit to coveralls over codeclimate is that coveralls will retain per-job metrics, so you can see that, for example, coverage on JRuby is 79.03% but on MRI it's 86.87%.

If @hoxworth sets up the codeclimate stuffs from #141, I'll swap this PR over to use that instead.