First attempt on using Travis CI.

https://github.com/ariya/phantomjs/issues/11880
Travis CI: Ensure packages are cached for faster setup.

https://github.com/ariya/phantomjs/issues/11880
Travis CI: Fix the build directives.

https://github.com/ariya/phantomjs/issues/11880
Travis CI: Packages installation needs a priviliged access.

https://github.com/ariya/phantomjs/issues/11880
Travis CI: Ensure that the build script executable.

https://github.com/ariya/phantomjs/issues/11880
Travis CI: Silent build.

https://github.com/ariya/phantomjs/issues/11880