Use download_url() in language pack upgrader

Fixes #30.

src/WP_CLI/LanguagePackUpgrader.php

@@ -65,23 +65,14 @@ public function download_package( $package ) {
 			return $temp;
 		}
 
-		/*
-		 * Download to a temporary file because piping from cURL to tar is flaky
-		 * on MinGW (and probably in other environments too).
-		 */
-		$headers = array( 'Accept' => 'application/json' );
-		$options = array(
-			'timeout'  => 600,  // 10 minutes ought to be enough for everybody.
-			'filename' => $temp
-		);
-
 		$this->skin->feedback( 'downloading_package', $package );
 
-		/** @var \Requests_Response|null $req */
-		$req = Utils\http_request( 'GET', $package, null, $headers, $options );
-		if ( null !== $req && $req->status_code !== 200 ) {
-			return new \WP_Error( 'download_failed', $this->strings['download_failed'] );
+		$temp = download_url( $package, 600 ); // 10 minutes ought to be enough for everybody.
+
+		if ( is_wp_error( $temp ) ) {
+			return $temp;
 		}
+
 		$cache->import( $cache_key, $temp );
 
 		return $temp;