Addressed warnings when reading from new files

There were warnings when reading extensions (trying to use e.g. README
and .gitignore as directories), and when reading update file.
https://github.com/FreshRSS/FreshRSS/issues/733

app/Controllers/updateController.php

@@ -28,7 +28,10 @@ class FreshRSS_update_Controller extends Minz_ActionController {
 			);
 		} elseif (file_exists(UPDATE_FILENAME)) {
 			// There is an update file to apply!
-			$version = file_get_contents(join_path(DATA_PATH, 'last_update.txt'));
+			$version = @file_get_contents(join_path(DATA_PATH, 'last_update.txt'));
+			if (empty($version)) {
+				$version = 'unknown';
+			}
 			$this->view->update_to_apply = true;
 			$this->view->message = array(
 				'status' => 'good',

lib/Minz/ExtensionManager.php

@@ -56,6 +56,9 @@ class Minz_ExtensionManager {
 
 		foreach ($list_potential_extensions as $ext_dir) {
 			$ext_pathname = EXTENSIONS_PATH . '/' . $ext_dir;
+			if (!is_dir($ext_pathname)) {
+				continue;
+			}
 			$metadata_filename = $ext_pathname . '/' . self::$ext_metaname;
 
 			// Try to load metadata file.