Merge pull request #77 from wp-cli/update/wp-cli-2-12

Require `wp-cli/wp-cli` v2.12

Author: mrsdizzie

composer.json

@@ -15,7 +15,7 @@
     },
     "require": {
         "php": ">=5.4",
-        "wp-cli/wp-cli": "^2"
+        "wp-cli/wp-cli": "^2.12"
     },
     "require-dev": {
         "wp-cli/wp-cli-tests": "^4"