Replace double quotes with single quotes
Remove therubyracer in favor of nodejs

therubyracer is known to be less memory-efficient than nodejs. the
JavaScript runtime now is native Node.js.
Update bundle
Use github as source of `guard-uglify`

The new version of `guard-uglify` was not pushed to rubygems.org.
Change guard-uglify output to avoid infinite loop

If the output is the same as the input, guard-uglify will constantly
uglify the file because it detects a file change after it uglifies
the file itself.