Add `es6-promise` as dependency as opposed to devDependency

package.json

@@ -74,7 +74,6 @@
     "chalk": "1.1.1",
     "cssnano": "3.6.2",
     "disc": "1.3.2",
-    "es6-promise": "3.1.2",
     "eslint": "2.7.0",
     "eslint-config-standard": "5.1.0",
     "eslint-plugin-promise": "1.1.0",
@@ -103,6 +102,7 @@
     "drag-drop": "2.11.0",
     "tus-js-client": "1.1.3",
     "whatwg-fetch": "1.0.0",
+    "es6-promise": "3.1.2",
     "yo-yo": "1.2.0"
   }
 }