Bump grunt-contrib-less to ~1.0.0

This updates Grunt's Less.js from v1.7.5 to v2.1.2.
Per [the Less v2 upgrade guide](http://lesscss.org/usage/#v2-upgrade-guide), the language is basically unchanged.
The main changes are to Less.js's API, so the follow-up task of upgrading the version of Less used in the Customizer will be a bit harder.
No changes to our `.less` files are required, and there are no changes to the resulting CSS.
CC: @mdo