Fix double bundling on file change

Karma already triggers a `deferredBundle()` if it detects a file change to a main file. A rebuild should only happen if watchify sends an update for a file **not** handled by the karma watcher. Since it's all or nothing, we only have to check until we find the first file that **is** handled by karma. (fwiw tests pass in browserify-7.0.2 branch)