Update dependency on phantomjs

The old module that pulled in phantomjs in the builds was old and wasn't working on
all platforms. This one seems like it is better supported.

Author: pontusmelke

package.json

@@ -41,7 +41,7 @@
     "jasmine-reporters": "^2.0.7",
     "merge-stream": "^1.0.0",
     "minimist": "^1.2.0",
-    "phantomjs2-ext": "^0.1.0",
+    "phantomjs-prebuilt": "^2.1.7 ",
     "run-sequence": "^1.1.4",
     "through2": "~2.0.0",
     "vinyl-buffer": "^1.0.0",