Issue 7

this pull request is comprised of changes that (hopefully) fix the travis ci build errors.

- the google gson dependency was rolled back to a more stable version.
- `mvn install:plugin` commands were added to the `before_install` handler in `.travis.yml`
- repository declarations (for supposedly blacklisted repositories) and build:plugin directives were removed from `pom.xml`
- the build failures listed in this pull request are not related to our codebase or our `pom.xml` being malformed. inspection of the [builds](https://travis-ci.org/skibz/xforms-service/builds) shows that it sometimes hangs up on fetching a specific dependency from the maven central repo, regardless of jdk version. sometimes it manages to fetch, sometimes it doesn't. -.-'