upgrade less-middleware, use new middleware signature
add autoprefixer dependency
use autoprefixer to post-process LESS
before testing on travis, update npm

npm for node 0.8.x (which we currently run on) doesn't support caret
notation (`^`) in dependency definitions. autoprefixer introduces these
kinds of definitions in its own dependencies.

https://github.com/travis-ci/travis-ci/issues/2076 ->
https://github.com/jashkenas/underscore/commit/4de07c008e53f2d4d8411fc315662496e80a8ea1
Merge pull request #46 from twinlabs/introduce-autoprefixer

Introduce autoprefixer