Re-written gulpfile for gulp 4

- use the new, simpler format
- sourcemap and watch support is built in, so these plugins can be removed
- remove js filter - not sure what that did :/
- simplified task structure

package.json

@@ -48,12 +48,9 @@
         "cssnano": "4.1.10",
         "gscan": "2.2.1",
         "gulp": "4.0.0",
-        "gulp-filter": "5.1.0",
         "gulp-livereload": "4.0.1",
         "gulp-postcss": "8.0.0",
-        "gulp-sourcemaps": "2.6.5",
         "gulp-uglify": "3.0.2",
-        "gulp-watch": "5.0.1",
         "gulp-zip": "4.2.0",
         "postcss-color-function": "4.0.1",
         "postcss-custom-properties": "8.0.9",