Tests running under Travis CI 

I wanted to run some tests that use Predis\Async under Travis but phpiredis wouldn't build there. There were some ZTS parameters missing and differences between a forward declaration and function definition. I've made changes that allow it build now, so I figured I may as well add a .travis.yml for phpiredis itself. I don't know much about the Zend API so of course please let me know if I've messed up.