Don’t depend on package locations, be more liberal about versions

This pull request solves the problem that occurs when *referee, sinon,* etc. are not installed in the `node_folders` folder below *karma-referee,* but as siblings to it. As it seems, newer npm versions might do this, or `npm dedupe`.

The problem is solved by finding files using node’s algorithm (provided through the *resolve* module).

I also made the allowed versions of *referee, sinon,* and *referee-sinon* more liberal by searching for the minimal version that works. This way, *karma-referee* integrates more nicely into existing setups.

Tests pass, jshint passes, and I’ve verified that it works.