Give serve preference to files in .tmp

From my testing, the browserSync server prefers files in app over files in .tmp when they have the same name. .tmp files should be preferred, since anything putting a file into .tmp is processing it in some way (e.g., a main.css file that's autoprefixed into .tmp, and an index.html that contains some templating that needs to be precompiled and output into .tmp).

I haven't played with the starter kit too much, so excuse me if I missed something here.