Bump version to 0.6.0 for release (#53)

Author: michaelbausor

src/AgentHeaderDescriptor.php

@@ -42,7 +42,7 @@ class AgentHeaderDescriptor
     const AGENT_HEADER_KEY = 'x-goog-api-client';
     // TODO(michaelbausor): include bumping this version in a streamlined
     // release process. Issue: https://github.com/googleapis/gax-php/issues/48
-    const GAX_VERSION = '0.6.0-dev';
+    const GAX_VERSION = '0.6.0';
 
     private $clientName;
     private $clientVersion;