tools: enable/add additional eslint rules

Enables the following rules:

- no-undef: Valuable rule to error on usage of undefined variables
- require-buffer: Custom rule that forbids usage of the global Buffer
  inside lib/ because of REPL issues.

PR-URL: https://github.com/nodejs/io.js/pull/1794


Reviewed-By: Trevor Norris <trev.norris@gmail.com>