Run tests against dev branch

Author: danielbachhuber

.travis.yml

@@ -37,7 +37,7 @@ before_install:
   - phpenv config-rm xdebug.ini
 
 install:
-  - composer require wp-cli/wp-cli:dev-master
+  - composer require wp-cli/wp-cli:dev-3728-cache-command
   - composer install
   - bash bin/install-package-tests.sh