Fix call for README file in repository.

Author: stloyd

lib/Github/Api/Repository/Contents.php

@@ -23,7 +23,9 @@ class Contents extends AbstractApi
      */
     public function readme($username, $repository, $reference = null)
     {
-        return $this->show($username, $repository, 'readme', $reference);
+        return $this->get('repos/'.urlencode($username).'/'.urlencode($repository).'/readme', array(
+            'ref' => $reference
+        ));
     }
 
     /**

test/Github/Tests/Api/Repository/ContentsTest.php

@@ -37,7 +37,7 @@ public function shouldShowReadme()
         $api = $this->getApiMock();
         $api->expects($this->once())
             ->method('get')
-            ->with('repos/KnpLabs/php-github-api/contents/readme', array('ref' => null))
+            ->with('repos/KnpLabs/php-github-api/readme', array('ref' => null))
             ->will($this->returnValue($expectedValue));
 
         $this->assertEquals($expectedValue, $api->readme('KnpLabs', 'php-github-api'));