Suggestion to support FindBugs version 2

Hi, I am interested in #11 and here is a suggestion.

I think that you do not have to maintain 2 branches, because FindBugs does not change its interface so much. Your TODO is executing test with both of 1.3.9 and 2.0.1.

See the diff of this pull request, there is no change in the code but it can work with both of 1.3.9 and 2.0.1.
I also changed the scope of dependency to FindBugs: it can be `provided` because user (FindBugs plugin) will depend on FindBugs and this dependency (user to FindBugs) will hide your dependency (test-driven-detectors4findbugs to FindBugs).

Check result of build at Travis CI:

* [Build at Travis](https://travis-ci.org/eller86/test-driven-detectors4findbugs/builds/8807939)
    * [Build with version 1.3.9 on OpenJDK7 (passed)](https://travis-ci.org/eller86/test-driven-detectors4findbugs/jobs/8807941)
    * [Build with version 2.0.1 on OpenJDK7 (passed)](https://travis-ci.org/eller86/test-driven-detectors4findbugs/jobs/8807943)

See also:

* Document for user of Travis CI
    * [Set environment variables](http://about.travis-ci.org/docs/user/build-configuration/#Set-environment-variables)