Enable TravisCI to deploy SNAPSHOT builds

Fixes #440 by providing TravisCI's build environment with an encrypted username and password to use when deploying to Sonatype's snapshot repository. The username and password being used are from the new [continuous integration account set up by the Sonatype team](https://issues.sonatype.org/browse/OSSRH-12020).

The values are [encrypted](http://docs.travis-ci.com/user/encryption-keys/) using the keys for the dropwizard/dropwizard repository. They will not work (by design) for any forks of the project, preventing others from getting access to the raw credentials. We should take care not to accept any subsequent pull requests that may expose those credentials (i.e. by `echo`'ing them to the console).

I've tested these changes in a fork, including forking my fork to create a pull request and verify that the pull request doesn't subsequently deploy its changes. Allowing a pull request to deploy a SNAPSHOT would expose us to nefarious individuals seeking to taint the SNAPSHOT repo with bad code. While TravisCI prevents this by not making encrypted values available to builds based off of pull requests, I've added an additional check to the TravisCI build to skip the deploy step altogether in a pull request build.