Rebuild when any .js file in src/ changes.

The build/dep.mk approach to triggering builds requires
approximately 2x more time per build than the build.
In general we do not include any files in src/ that we
do not compile into traceur.
Therefore simply depending upon src/**/*.js will make
builds faster without triggering extra builds

Review URL: https://github.com/google/traceur-compiler/pull/649

Closes #649.