Support WP-CLI 2+

Author: danielbachhuber

composer.json

@@ -13,11 +13,11 @@
     },
     "require": {
         "php": ">=5.4",
-        "wp-cli/wp-cli": "2"
+        "wp-cli/wp-cli": "^2"
     },
     "require-dev": {
         "behat/behat": "~2.5",
-        "wp-cli/wp-cli-tests": "2"
+        "wp-cli/wp-cli-tests": "^2"
     },
     "config": {
         "platform": {