Use output of --log-json to calculate test results instead of parsing stdout from phpunit

Using different `--printer` classes for phpunit results in output that can not be consistently parsed. 

This PR parses the output of `--log-json`(http://phpunit.de/manual/3.7/en/logging.html) instead.

Everytime we execute `phpunit`, this PR now passes in a `--log-json` flag that tells PHPUnit to also write logging output to the specified temp file. When complete, we can then read and parse that JSON.

I've added specs for the new `LogReader` class as well as sample output fixtures and php unit tests to generate them. I tried to make as minimal changes in `runner_spec.rb` as possible. If accepted, I can remove all the specs that assert things about PHPUnit's STDOUT (STDOUT can be ignored since the JSON logging gives all the needed information).

On my environment I had to fight with the `Gemfile` to get `pry` and `guard` to play nice. You can probably revert 765d8d9.