Support gulp-sourcemaps

I believe this block should not be removed, together with the other if(ret.sourceMap) block that actually writes the sourceMap. The additional !file.sourceMap constraint suggested in the first version of the pullrequest should be reinstantiated and also added to the other block. This would maintain BC and make this module not dependent on the gulp-sourcemaps module in your gulpfile. This is also what breaks the current tests.

@sindresorhus : whats your take on this?