Fix module names for "register" implementation

When compiling with "register" implementation, we found that modules names only contains file names. Ex :

    src/common/filter.js -> System.register("filter")

In contrario, [karma's traceur preprocessor](karma-runner/karma-traceur-preprocessor) produce the expected result:

    src/common/filter.js -> System.register("src/common/filter")

This tiny pull request fix this, without changing the current behaviour of others implementaions ('amd', 'commonjs')
I'v just tried in live, but `file.relative` does not work. 

I'll investigate in the Traceur code to explain why it fails, but you [already knows](https://github.com/sindresorhus/gulp-traceur/issues/15) there are no documentation relative to this module implementation :).

I'll add a test also.