Add missing webpack-dev-server dev dep

Author: Brandon Dail

package.json

@@ -42,6 +42,7 @@
     "jest-cli": "^15.1.1",
     "radium": "^0.18.1",
     "rimraf": "^2.5.4",
-    "webpack": "^2.1.0-beta.25"
+    "webpack": "^2.1.0-beta.25",
+    "webpack-dev-server": "^1.16.1"
   }
 }