chore(package): update coveralls to version 2.13.1

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -9,7 +9,7 @@
   "bugs": "https://github.com/static-dev/source-loader/issues",
   "devDependencies": {
     "ava": "^0.19.0",
-    "coveralls": "2.x",
+    "coveralls": "^2.13.1",
     "nyc": "10.2.0",
     "standard": "10.0.0",
     "webpack": "^1.13.0"