chore(package): update es6-promise to version 4.2.6

Closes #275

Author: greenkeeper[bot]

package.json

@@ -41,7 +41,7 @@
     "codecov": "^3.0.0",
     "coffee-script": "^1.12.1",
     "coffeelint": "^1.15.7",
-    "es6-promise": "4.2.0",
+    "es6-promise": "4.2.6",
     "eslint": "^4.1.0",
     "eslint-config-standard": "^11.0.0-beta.0",
     "eslint-plugin-promise": "^4.0.0",