[INTERNAL] lock grunt-eslint version to 2.0.0

grunt-eslint 2.1.0 brings eslint 0.10.x which has breaking changes.
For now, just lock the version to use eslint 0.9.x again.

Change-Id: Ie1abc52ca9810759c6229beca795a01c64994db0
Merge "[INTERNAL] lock grunt-eslint version to 2.0.0"