feat(gpm): send PHP version with index requests

Lets the GPM server return PHP-aware compat fallbacks: when a plugin's
latest release requires a newer PHP than the client can run, the server
now substitutes the most recent compatible older release instead of
serving an uninstallable latest.

Author: rhukster

system/src/Grav/Common/GPM/Remote/AbstractPackageCollection.php

@@ -49,7 +49,7 @@ public function __construct($repository = null, $refresh = false, $callback = nu
         $cache_dir = Grav::instance()['locator']->findResource('cache://gpm', true, true);
         $this->cache = new FilesystemCache($cache_dir);
 
-        $this->repository = $repository . '?v=' . GRAV_VERSION . '&' . $channel . '=1';
+        $this->repository = $repository . '?v=' . GRAV_VERSION . '&php=' . PHP_VERSION . '&' . $channel . '=1';
         $this->raw        = $this->cache->fetch(md5($this->repository));
 
         $this->fetch($refresh, $callback);

system/src/Grav/Common/GPM/Remote/GravCore.php

@@ -43,7 +43,7 @@ public function __construct($refresh = false, $callback = null)
         $channel = Grav::instance()['config']->get('system.gpm.releases', 'stable');
         $cache_dir   = Grav::instance()['locator']->findResource('cache://gpm', true, true);
         $this->cache = new FilesystemCache($cache_dir);
-        $this->repository .= '?v=' . GRAV_VERSION . '&' . $channel . '=1';
+        $this->repository .= '?v=' . GRAV_VERSION . '&php=' . PHP_VERSION . '&' . $channel . '=1';
         $this->raw = $this->cache->fetch(md5($this->repository));
 
         $this->fetch($refresh, $callback);