Upgrade lesscss to 1.7.5.

During Review Board 2.0 development, we upgraded to less 1.5.1, which
had some changes we provided to them needed for our extension support.
Many improvements have since gone in that we'd like to take advantage
of.

This change bumps us up to 1.7.5. This is backwards-compatible with
1.5.1, and shouldn't negatively impact any extensions or the site.