Swapped fetch polyfill

.travis.yml

@@ -24,7 +24,7 @@ before_install:
   - java -jar selenium.jar &
   - npm install -g nightwatch
   - nightwatch -v
-  - npm install whatwg-fetch
+  - npm install node-fetch --no-save
 
 script:
   - cargo build --verbose

test/nightwatch/globals.js

@@ -1,7 +1,7 @@
 module.exports = {
 	before : function(cb) {
 		console.log("Requiring fetch polyfill");
-		fetch = require('whatwg-fetch').fetch;
+		fetch = require('node-fetch').fetch;
 		cb();
 	}
 };