Stick to jscs 1.6.x

1.7.x crashes on our code.
https://github.com/jscs-dev/node-jscs/issues/684

It is merged, but still not released, so we'll use 1.6 for now.
Support for source maps in coverage reports
Merge pull request #71 from bem/feature/sourcemap

Support for source maps in coverage reports
gm -> png-img
README fixed