Use the api to get file contents instead of raw.github.com

raw.github does not like the access_token query param

src/Composer/Repository/Vcs/GitHubDriver.php

@@ -128,8 +128,11 @@ class GitHubDriver extends VcsDriver
 
         if (!isset($this->infoCache[$identifier])) {
             try {
-                $resource = 'https://raw.github.com/'.$this->owner.'/'.$this->repository.'/'.$identifier.'/composer.json';
-                $composer = $this->getContents($resource);
+                $resource = 'https://api.github.com/repos/'.$this->owner.'/'.$this->repository.'/contents/composer.json?ref='.urlencode($identifier);
+                $composer = JsonFile::parseJson($this->getContents($resource));
+                if (empty($composer['content']) || $composer['encoding'] !== 'base64' || !($composer = base64_decode($composer['content']))) {
+                    throw new \RuntimeException('Could not retrieve composer.json from '.$resource);
+                }
             } catch (TransportException $e) {
                 if (404 !== $e->getCode()) {
                     throw $e;

tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php

@@ -198,8 +198,8 @@ class GitHubDriverTest extends \PHPUnit_Framework_TestCase
 
         $remoteFilesystem->expects($this->at(1))
             ->method('getContents')
-            ->with($this->equalTo('github.com'), $this->equalTo('https://raw.github.com/composer/packagist/feature/3.2-foo/composer.json'), $this->equalTo(false))
-            ->will($this->returnValue('{"support": {"source": "'.$repoUrl.'" }}'));
+            ->with($this->equalTo('github.com'), $this->equalTo('https://api.github.com/repos/composer/packagist/contents/composer.json?ref=feature%2F3.2-foo'), $this->equalTo(false))
+            ->will($this->returnValue('{"encoding":"base64","content":"'.base64_encode('{"support": {"source": "'.$repoUrl.'" }}').'"}'));
 
         $remoteFilesystem->expects($this->at(2))
             ->method('getContents')