Downgrade express to the stable version

Express 4.3.2, which we use, has a vulnerability. The stable version
should hopefully not have this vulnerability (the testing version is
still in alpha, so probably not ideal for us to switch to it at this
point in time).

package.json

@@ -58,7 +58,7 @@
     "debug": "^2.2.0",
     "dotenv": "^1.2.0",
     "errorhandler": "^1.4.2",
-    "express": "^4.3.2",
+    "express": "~4.14.0",
     "express-state": "^1.2.0",
     "express-useragent": "^0.2.0",
     "extract-text-webpack-plugin": "2.0.0-beta.3",