tools: set eslint comma-spacing to 'warn'

Certain cases with comments inside arrays or object literals fail to
pass eslint's comma-spacing rule. This change sets the comma-spacing
rule to the 'warn' level.

Once https://github.com/eslint/eslint/issues/2408 is resolved and
released, this rule should be set back to 'error' level.

PR-URL: https://github.com/iojs/io.js/pull/1672


Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>