Does not run on Travis CI

Hello,

I'd like to know if it is possible to run acceptance tests on Travis CI. By seeing the `.travis.yml` file on the project root directory, I suppose it is possible.

It looks like you are using the deprecated wp-cli installer (https://github.com/wp-cli/wp-cli/wiki/Alternative-Install-Methods#installersh-deprecated), which causes an error. Using the phar file of the wp-cli may be an option.

Apart from it, I'm not sure how it runs on Travis. I'd like to see a working example. Can you update it to make the tests run on Travis? Thank you!