Remove stream subscription to write events on empty buffer.

This commit fixes issue #5. The reported bug was introduced in v0.2.0
due to a breaking change in react/event-loop that was not properly
addressed in Predis\Async after bumping the above mentioned dependency.

Props to @bixuehujin for spotting the actual cause of the issue.

lib/Predis/Async/Client.php

@@ -36,7 +36,7 @@
  */
 class Client
 {
-    const VERSION = '0.2.0';
+    const VERSION = '0.2.1-dev';
 
     protected $profile;
     protected $connection;

lib/Predis/Async/Connection/StreamConnection.php

@@ -374,11 +374,13 @@ protected function getIdentifier()
      */
     public function write()
     {
+        $socket = $this->getResource();
+
         if ($this->buffer->isEmpty()) {
-            return false;
+            $this->loop->removeWriteStream($socket);
+            return;
         }
 
-        $socket = $this->getResource();
         $buffer = $this->buffer->read(4096);
 
         if (-1 === $ret = @stream_socket_sendto($socket, $buffer)) {