Bug 1395176 - Add eslint rule that forbids use of var. r=automatedtester

MozReview-Commit-ID: GvuSbH6EHea

--HG--
extra : rebase_source : 4093bf49449a6537ed0b428d93767bf6906ba8ee

testing/marionette/.eslintrc.js

@@ -17,6 +17,7 @@ module.exports = {
     "no-fallthrough": "error",
     "no-new-object": "error",
     "no-undef-init": "error",
+    "no-var": "error",
     "object-curly-spacing": ["error", "never"],
     "semi": "error",
   }