Improve Travis Continuous Integration

[fred-wang]

I've connected the TeXZilla repository to travis. After several attempts, I ended up doing "build only".

https://travis-ci.org/fred-wang/TeXZilla/builds

Ideally we should also:

1. run the tests with nodejs
2. run the tests with phantomjs
3. run the tests with slimerjs (not available as a package for Ubuntu precise)
4. run "make minify" (closure-compiler not available as a package for Ubuntu precise, but is in more recent version)