chore(package): update grunt-gh-pages to version 3.1.0

Closes #54

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
     "grunt-coffeelint": "0.0.16",
     "grunt-contrib-connect": "^1.0.0",
     "grunt-exec": "^2.0.0",
-    "grunt-gh-pages": "^2.0.0",
+    "grunt-gh-pages": "^3.1.0",
     "grunt-mocha-test": "^0.13.2",
     "mocha": "^4.0.0",
     "phantomjs-prebuilt": "^2.1.12",