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

Travis has been failing for Node 0.8 and 0.11 due to 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.