Make the CI fail on ESLint warnings

Many of our ESLint rules are configured as warning rather than errors
due to the level used in the imported configuration, but we still want
to enforce them.

Author: stof

package.json

@@ -7,7 +7,7 @@
     "test": "yarn run test:main && yarn run test:persistent-cache",
     "test:main": "mocha --reporter spec test --recursive --ignore test/persistent-cache/*",
     "test:persistent-cache": "node run-persistent-tests",
-    "lint": "eslint lib test index.js .eslintrc.js --report-unused-disable-directives",
+    "lint": "eslint lib test index.js .eslintrc.js --report-unused-disable-directives --max-warnings=0",
     "travis:lint": "yarn run lint"
   },
   "bin": {