Add test-unit as dev dependency

## What? Why?
As a prequel to updating the ```public_suffix``` gem, gotta get the test suite to run!

The test suite doesn't currently run on Ruby 2 since TestUnit has been deprecated in favor of MiniTest. This PR adds [TestUnit](https://github.com/test-unit/test-unit) as a gem.

## Additional Deploy Steps?
None


## How was it tested?
To replicate issue:
* Checkout master
* Switch to Ruby 2 something: ```rvm use ruby-2.0.0-p481```
* Install dependencies: ```bundle install --clean```
* Verify test suite fails: ```bundle exec rake```

Now:
* Checkout my branch
* Switch to Ruby 2 something: ```rvm use ruby-2.0.0-p481```
* Install dependencies: ```bundle install --clean```
* Verify test suite runs as expected: ```bundle exec rake```

cc @IamDavidovich @splittingred @jlleblanc @charrisbc