tools: enable prefer-const eslint rule

Description from: http://eslint.org/docs/rules/prefer-const.html

If a variable is never modified, using the `const` declaration is
better. `const` declaration tells readers, "this variable is never
modified," reducing cognitive load and improving maintainability.

Refer: https://github.com/nodejs/node/issues/3118
PR-URL: https://github.com/nodejs/node/pull/3152


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>