Make DropwizardAppRule extend ExternalResource

ExternalResource is a junit rule that standardizes the try-before-apply-finally-after pattern already used in the original code implmenting TestRule.

By using the ExternalResource, everything works as before, but you can now use before() and after() hooks as extension points.
In my case, I would run flyway db initialization in my own rule extending DwAppRule by using super.before();.
Merge pull request #760 from jangalinski/appRule_as_externalResource

Make DropwizardAppRule extend JUnit's ExternalResource