🐛 Do not exclude assets/css from built zips

no issue

https://github.com/TryGhost/Casper/pull/403 added optimisations for reducing size of zip files but unfortunately it also stripped out `assets/css` which means that it's no longer possible to download the theme from the admin panel and use it for development.

- remove `assets/css` from the ignored files in the `zip` task

gulpfile.js

@@ -59,8 +59,7 @@ gulp.task('zip', ['css'], function() {
     return gulp.src([
         '**',
         '!node_modules', '!node_modules/**',
-        '!dist', '!dist/**',
-        '!assets/css', '!assets/css/**',
+        '!dist', '!dist/**'
     ])
         .pipe(zip(filename))
         .pipe(gulp.dest(targetDir));