Add Travis CI building system

Should fix #140. The list of packages to build still requires discussion. However, be aware that Travis CI cannot build all packages at once, or it will end up like [this](https://travis-ci.org/TimothyGu/mxe/jobs/13556477#L386).

There are some comments in the .travis.yml file. They are to be enabled after Travis CI is tested enough that it works. They are disabled right now because it will either spam the mailing list, or it will mess up my own repo.