fix: fix crash when installing with yarn

The package phantomjs is actually just an older version of
phantomjs-prebuilt. This older version has a bug which makes it crash
when trying to install with yarn.

See: yarnpkg/yarn#1016 (comment)

In fact, since karma-phantomjs-launcher already installs
phantomjs-prebuilt (and we don't actually use PhantomJS other than
through that module), we don't also need to depend on it at all.

package.json

@@ -252,9 +252,7 @@
     "karma-sourcemap-loader": "^0.3.5",
     "karma-webpack": "^1.7.0",
     "mocha": "^2.4.5",
-    "phantomjs": "^1.9.18",
     "phantomjs-polyfill": "0.0.2",
-    "phantomjs-prebuilt": "^2.1.7",
     "redux-devtools": "^3.3.1",
     "redux-devtools-dock-monitor": "^1.1.1",
     "redux-devtools-log-monitor": "^1.0.11",