configure Travis CI services

This pull request adds `.travis.yml` file containing a configuration for [Travis CI](https://travis-ci.org/) testing of the module.

It depends on the previous testing-related request (#170) and contains a commit from it because otherwise the testing would fail.

This configuration is the third step in the process of [getting started](http://about.travis-ci.org/docs/user/getting-started/) with Travis CI. You may be willing to complete that process in order to see (almost immediately) if any of the following commits (and pull requests) pass the tests (`npm test`) properly.

I have completed that process for my fork and thus you may see the “Good to merge” message from Travis CI here.

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