Hook up Travis CI to run tests on pull requests

[Opal Mitchell]

Once we have the npm test command working (see #33) we can hook up Travis CI so tests are run on pull requests. If we add a ".travis.yaml" file to the root of the project with the following contents, it should work.

language: node_js
sudo: false
script:
  - npm test

We may have to add a ".nvmrc" file to the root of the project with the following contents to tell Travis CI what version of Node to use.

4.4.0