Fix mvn issuing 5k "we have a duplicate" warnings.

dropwizard-guice depends on javassist 3.12.1, while
hibernate-entitymanager dpends on javassist 3.15.0. This lead to maven
issuing about 5k duplicate class warnings. This commit excludes the
older version of javassist, preventing those warnings.
Merge pull request #2 from sopoforic/fix-duplicate-warnings

Fix mvn issuing 5k "we have a duplicate" warnings.