docs(CONTRIBUTING.md): Fixing Google Closure I18N library link (old link returns 404)

Request Type: docs

How to reproduce: 

Component(s): 

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**



**Other Comments:**

The old seems to link to the source code of I18N. Found the same folder on their new Github repo.

"Closure Library's source repository has moved to GitHub.", https://code.google.com/p/closure-library/