use webpack-dev-server 2.7.1 to fix npm start (#285)

webpack/webpack-dev-server#1101 (comment)

Author: machenmusik

package.json

@@ -14,7 +14,7 @@
     "mocha": "^3.2.0",
     "semver": "^5.3.0",
     "webpack": "^2.6.1",
-    "webpack-dev-server": "^2.4.5"
+    "webpack-dev-server": "2.7.1"
   },
   "main": "src/node-entry",
   "keywords": [
@@ -32,4 +32,4 @@
   "bugs": {
     "url": "https://github.com/googlevr/webvr-polyfill/issues"
   }
-}
+}