Fix build error for TravisCI in Node.js v0.8

Because the project now uses the "carrot operator" to specify some of
its dependencies, NPM must be explicitly updated in the TravisCI
environment before the build can succeed there.

Sources:

- https://github.com/npm/npm/wiki/Troubleshooting#travis-projects-using-08-cant-upgrade-to-npm-2
- https://github.com/travis-ci/travis-ci/issues/1785#issuecomment-31253761