Add .travis.yml for travis-ci.org testing

http://docs.travis-ci.com/user/getting-started/

If we link travis-ci.org to https://github.com/alibaba/tengine,
we can perform continuous testing against new commits to ensure
they do not break the build. travis-ci.org is a free service.

This commit provides an appropriate configuration file to tell
travis-ci.org about tengine and how we build it from source.