minor symfony#25833 fix the Composer API being used (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
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fix the Composer API being used

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Commits
-------

a986efe fix the Composer API being used

Author: fabpot

.github/build-packages.php

@@ -49,8 +49,8 @@
 
     $packages[$package->name][$package->version] = $package;
 
-    $versions = file_get_contents('https://packagist.org/packages/'.$package->name.'.json');
-    $versions = json_decode($versions)->package->versions;
+    $versions = file_get_contents('https://packagist.org/p/'.$package->name.'.json');
+    $versions = json_decode($versions)->packages->{$package->name};
 
     if ($package->version === str_replace('-dev', '.x-dev', $versions->{'dev-master'}->extra->{'branch-alias'}->{'dev-master'})) {
         unset($versions->{'dev-master'});