Ensure the new loop follows the standard set by @Ocramius in reactphp…

…#167, pointed out by @clue at reactphp#112 (comment)

src/ExtUvLoop.php

@@ -31,7 +31,7 @@ final class ExtUvLoop implements LoopInterface
 
     public function __construct()
     {
-        if (!function_exists('uv_loop_new')) {
+        if (!\function_exists('uv_loop_new')) {
             throw new \BadMethodCallException('Cannot create LibUvLoop, ext-uv extension missing');
         }