Merge pull request #113 from minbaby/hotfix/tracingbuilder

fix if endpoint is null, than service name is invalid

src/Zipkin/TracingBuilder.php

@@ -152,7 +152,7 @@ public function build()
         if ($localEndpoint === null) {
             $localEndpoint = Endpoint::createFromGlobals();
             if ($this->localServiceName !== null) {
-                $localEndpoint->withServiceName($this->localServiceName);
+                $localEndpoint = $localEndpoint->withServiceName($this->localServiceName);
             }
         }