Fix to run 0.8-stable's tests

Before this PR, I couldn't run tests on 0.8-stable branch:
```
MiniTest::Unit::TestCase is now Minitest::Test. From /Users/tricknotes/.rbenv/versions/2.1.2/lib/ruby/2.1.0/test/unit/testcase.rb:8:in `<module:Unit>'
/Users/tricknotes/.rbenv/versions/2.1.2/lib/ruby/2.1.0/test/unit.rb:676:in `<class:Runner>': undefined method `_run_suite' for class `Test::Unit::Runner' (NameError)
```

Some tests with old ruby versions are failed yet, but current maintained version(1.9.3, 2.0.0) are passed on Travis CI.
see: https://travis-ci.org/tricknotes/active_model_serializers/builds/34693470