Fix flakiness with Gulp + Travis

Gulp was failing silently on Travis. I tracked this down to the task that
builds a custom, slimmed-down version of jQuery from source.

To fix this I’ve removed the task and replaced it with just `src`ing the
minified version of jQuery from `node_modules`.

Cons:
- A few more kb of JS

Pros:
- Less random, afternoon-chewing complexity

package.json

@@ -7,9 +7,9 @@
   },
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
-    "postinstall": "./node_modules/bower/bin/bower install && npm run build",
-    "build": "./node_modules/gulp/bin/gulp.js",
-    "watch": "./node_modules/gulp/bin/gulp.js watch"
+    "postinstall": "./node_modules/bower/bin/bower install",
+    "build": "gulp",
+    "watch": "gulp watch"
   },
   "repository": {
     "type": "git",