Try to fix Travis' karma failures do to error in js coverage report

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

package.json

@@ -24,7 +24,7 @@
   },
   "homepage": "https://github.com/nextcloud/mail",
   "devDependencies": {
-    "coveralls": "^2.11.14",
+    "coveralls": "^2.13.0",
     "grunt": "^1.0.1",
     "grunt-cli": "^0.1.13",
     "grunt-contrib-jshint": "^0.11.3",