ZipFileStorage: hardcode version jszip to 3.1.x

package.json

@@ -45,7 +45,7 @@
     "grunt-contrib-watch": "~0.5.3",
     "grunt-jslint": "~1.0.0",
     "lz-string": "^1.4.4",
-    "jszip": "",
+    "jszip": "3.1.x",
     "sinon": "~1.7.3",
     "jison": "~0.4.16",
     "connect-livereload": "~0.3.0",