Bump version to 4.0.0 (#547)

Major version bump as the preceeding PR actioned a major version bump of the
minimum supported php version for this package.

Author: token-cjg

VERSION

@@ -1 +1 @@
-3.1.1
+4.0.0

src/Zendesk/API/HttpClient.php

@@ -114,7 +114,7 @@
  */
 class HttpClient
 {
-    const VERSION = '3.1.1';
+    const VERSION = '4.0.0';
 
     use InstantiatorTrait;