travis: move init script to separate file
travis: move main script to separate file
travis: set WITH_RONN env variable

Also, tag behat tests that depend on the ronn gem.

We don't care too much about which WP version is installed, so run the
ronn tests only once.
Merge pull request #479 from wp-cli/behat-env

behat: separate tests that require the `ronn` gem