[TASK] Re-enable the travis_lint Rake task by default.

After the most recent travis-lint update, travis-lint recognizes Ruby 2.1,
causing the lint to not fail anymore on our .travis.yml.

So we can enable the rake task again.
Merge pull request #35 from braingourmets/task/travis-lint

[TASK] Re-enable the travis_lint Rake task by default.