Simplifying the DRb version of the runner

We're using guard-minitest with [spork-minitest][1]. Spork works when running just `spork` and then using `testdrb` to run individual files, but guard-minitest was broken. It kept saying

```
Running tests with args ["-r", "/Users/costro0001/.rvm/gems/ruby-1.9.2-p180-patched/gems/guard-minitest-0.5.0/lib/guard/minitest/runners/default_runner.rb", "-e
", "::GUARD_NOTIFY=true", "test/test_helper.rb", "./test/unit/raw_site_test.rb"]...
Exception encountered: #<LoadError: no such file to load -- -r>
```

It turns out that the [implementation of testdrb in spork-minitest][2] does not allow the `-r` or `-e` options in the way guard-minitest had been passing them in. Additionally, I found that the inclusion of test_helper/spec_helper was also unnecessary, as these files were already being included in the individual test that was being run.

I'm not sure why all of this extra machinery was in place. It seems strange that I just removed it. Let me know what you think.

  [1]: https://github.com/semaperepelitsa/spork-minitest
  [2]: https://github.com/semaperepelitsa/spork-minitest/blob/master/bin/testdrb