chore: fix modules/build.dart/pubspec

This was failing on Travis because the `pub install` would run before
copying of `pubspec.yml` happened. In fact, I don’t understand how this
worked at all because `gulp.dest` seems to be not forwarding files and
so anything after `gulp.dest` does not get called at all.

Here is the failing Travis build:
https://travis-ci.org/angular/angular/builds/40005692

This changes `modules/build.dart/pubspec` task to use `gulp-changed`
instead of a custom implementation and use a wrapper around `gulp.dest`
to forward files.