Add travis.yml file that will run smoke tests for each pull request.

Add a `.travis.yml` file to the repo root so travis-ci can run the smoke tests against all pull requests
and every time something is pushed to master.

[Travis](https://travis-ci.org/) is an open source continuous integration service for open source projects.
It integrates easily with github to provide build/test support when updates are made to the repo and
pull requests are proposed. For more information on the integration with github, take a look at
these links:

* http://blog.travis-ci.com/2012-09-04-pull-requests-just-got-even-more-awesome/
* http://docs.travis-ci.com/user/getting-started/
* http://docs.travis-ci.com/user/languages/c/

This file will run `util/buildRelease/smokeTest` with the `gcc` compiler (`clang` has a build issue at this time) and CHPL_DEVELOPER=true set in the environment. You can see an example build here:

https://travis-ci.org/chapel-lang/chapel/builds/34876795

### TODO

* [x] Update the chapel-lang/chapel repo settings to integrate with travis.
* [x] Gather consensus among chapel-developers about what this should be.
* [x] Decide if CHPL_DEVELOPER should be set, unset, or both.
What's the problem with clang? Does it affect only Linux?