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.
If you link travis-ci.org to the github account, it will watch for commits. When someone pushes a commit, it will download the source and build with these options. If the build fails, it will tell the committer that their change broke the software (via email) so they know.

These options are for the virtual machines at travis-ci.org. I just specified they should build with jemalloc, but of course that is an option and we can choose not to do it. These options do not affect end-users, only the testing VM at travis-ci.org.