fix issue #251 - Using $this not in object context

src/Composer/Repository/Vcs/HgDriver.php

@@ -185,7 +185,8 @@ class HgDriver extends VcsDriver implements VcsDriverInterface
             return false;
         }
 
-        $exit = $this->process->execute(sprintf('cd %s && hg identify %s', escapeshellarg(sys_get_temp_dir()), escapeshellarg($url)), $ignored);
+        $processExecutor = new ProcessExecutor();
+        $exit = $processExecutor->process->execute(sprintf('cd %s && hg identify %s', escapeshellarg(sys_get_temp_dir()), escapeshellarg($url)), $ignored);
         return $exit === 0;
     }
 }