Less peer dependency

[BarryThePenguin]

There is a current issue open in less/less.js#2896 for the latest release 2.7.0

When using gulp-less npm installs less@2.7.0 as a dependency.

This PR proposes changing less to be a peer-dependency.

I'm not sure if the range I've specified is the right approach ie "less": ">=2.x". This may need changing to "less": "<3" or something everyone can agree on..

There may be other finer points to this that I'm missing, though I did see this suggested in #183

Also note, this may need a major version bump as people will need to have less as an explicit dependency.

[wolfy1339]

This wouldn't really help... It would let users install all 2.x releases of less, which would include 2.7.0 that has that bug with sourcemaps.
If you look at my PR #238, I explicitly ignored 2.7.0