Merge pull request #75 from opensheetmusicdisplay/greenkeeper-es6-promise-4.0.5

Update es6-promise to version 4.0.5 🚀

package.json

@@ -41,7 +41,7 @@
   },
   "homepage": "http://opensheetmusicdisplay.org",
   "dependencies": {
-    "es6-promise": "^3.2.1",
+    "es6-promise": "^4.0.5",
     "jszip": "^3.0.0",
     "loglevel": "^1.4.1",
     "shortid": "^2.2.6",