Just use require-dev wp-cli/wp-cli:* for the mo.

Author: gitlost

composer.json

@@ -17,14 +17,15 @@
     "minimum-stability": "dev",
     "prefer-stable": true,
     "autoload": {
-        "psr-4": {"": "src/"},
+        "psr-4": {
+			"": "src/"
+		},
         "files": [ "db-command.php" ]
     },
-    "require": {
-        "wp-cli/wp-cli": ">=1.5.0"
-    },
+    "require": {},
     "require-dev": {
-        "behat/behat": "~2.5"
+        "behat/behat": "~2.5",
+        "wp-cli/wp-cli": "*"
     },
     "extra": {
         "branch-alias": {