Fixed gulp.js linting errors
Fixed gulp.js linting errors

- Removed unnecessary semicolons
- Used path.join() instead of `+` to join paths
- Watch the digitalmarketplace-frameworks directory (bugfix)
- Some small other tweaks

alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend