Make watchify a regular dependency and downgrade

As described in #9 watchify v1 introduces breaking changes. I also don't think it's a peerDep. To me peers are used by frameworks — things where the framework calls your module and not the other way around. So while a browserify transform is definitely a peer to browserify I don't see watchify as a peer to karma-bro. Especially because browserify is not a peer to watchify. If the idea is to allow a user to use a single browserify module across karma-bro and a build tool, that's not possible. 

Closes #9