Rails 4

@mikeycgto among others.

This is an upgrade of RankedModel to support Rails 4 with all test passing. Notable is that this uses `database_cleaner` instead of `rspec-rails` to flush database records.

This adds tests on Travis as well.

You can run the tests for a given version of Rails with:

```
ACTIVERECORD_VERSION="4.0.0" bundle update
ACTIVERECORD_VERSION="4.0.0" bundle exec rake
```

Everything looks clean on travis and in the tests for this PR. I'll leave it open for a bit them merge and bump a gem version.

Huzzah Rails 4!