Updated PR#68 to also change the file reference inside of a source map file to point to file revisioned source file; updates a test

Updates pull-request #68, (fixes #62)

I thought the file reference in the source map file should point back to the file it's mapping.

Doing so, my code triggered the test `'should use same revision as .js source for the .map'` to fail. I refactored it in a way that I think should be fine?

This code also introduces NodeJS Path Module to the tests.
This will fail when the `sourceMappingURL` is a relative path, not just a filename.

Use a regex to only replace the filename.