Remove gulp-concat-sourcemap plugin from blacklist

`gulp-concat-sourcemap` is not duplicate of `gulp-concat`.
`gulp-concat` can be used to concat all types of files.
`gulp-concat-sourcemap` is used to concat `.js` and `.css` files that need to have sourcemaps.

This functionality may be useful for everyone who wants to be able to debug resulting JS and CSS, and `gulp-concat` doesn't have such abilities. Corresponding issue was opened in wearefractal/gulp-concat#27 but it's not likely this functionality will be merged, so let's leave this plugin separately at least until any decision about merging will be made.