Add a before_install directive to .travis.yml to update npm

Travis has been failing for Node 0.6 and 0.8 (see https://github.com/npm/npm/issues/4849 and https://github.com/npm/npm/issues/5906)

As per the directions, adding the directive given [here](https://github.com/npm/npm/wiki/Troubleshooting#travis-projects-using-08-cant-upgrade-to-npm-2) to update NPM on travis.

While 0.6 is no longer supported by NPM, NPM version 1.3.26 seems to handle both carets (`^`) and tildes (`~`).

This should fix travis breakage.