Update dependency gulp-sourcemaps

Unpin `gulp-sourcemaps` from `1.6.0` and update it to `^2.2.1`.

This version was pinned because of a bug introduced in `v1.7.0` (PR
 #153, issue #230). The bug was resolved in `v1.7.2` (PR #235)

See:
- https://github.com/floridoo/gulp-sourcemaps/issues/230
- https://github.com/floridoo/gulp-sourcemaps/pull/153
- https://github.com/floridoo/gulp-sourcemaps/pull/235