Test only changed things

[thefinn93]

Currently we test the entire repo, but it might be beneficial to have a way to detect what has changed for eg a pull request and only test those files. This way if a peer that fails the tests somehow get's committed, future pull requests don't fail. Would also be good for warnings, which some things in the repo are expected to have.

[Kubuxu]

You can get list of changed files in travis by running:

git diff --name-only $TRAVIS_COMMIT_RANGE