tools: enable no-extra-semi rule in eslint

PR-URL: https://github.com/nodejs/node/pull/2205
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

.eslintrc

@@ -25,6 +25,8 @@ rules:
   no-dupe-keys: 2
   ## check duplicate switch-case
   no-duplicate-case: 2
+  ## disallow superfluous semicolons
+  no-extra-semi: 2
   ## disallow assignment of exceptional params
   no-ex-assign: 2
   ## disallow unreachable code