Made the default rake task run rspec
Made the default rake task run rspec

Right now travis isn't running any tests. At all. .travis.yml doesn't
specify any command to run, and so travis is doing the default action
(for a ruby project) and running ~bundle exec rake~. In the case of this
project, that runs test unit. Unfortunately, there aren't any test unit
tests, so nothing happens.

I've swapped out the test unit rake task for the default rspec one. This
should make tests run successfully on travis, but it's also good
practice for the default rake task to run tests.

jorgemanrubia/mailgun_rails,rgraff/mailgun_rails,rgraff/mailgun_rails,jorgemanrubia/mailgun_rails,jorgemanrubia/mailgun_rails,rgraff/mailgun_rails