Lock framework to v2.4.0+ (#140)

Lock framework to v2.4.0+

Author: schlessera

composer.json

@@ -13,7 +13,7 @@
     ],
     "require": {
         "composer/semver": "^1.4",
-        "wp-cli/wp-cli": "dev-master"
+        "wp-cli/wp-cli": "^2.4"
     },
     "require-dev": {
         "wp-cli/checksum-command": "^1 || ^2",