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