Travis badge teak (point to master).

Author: pq

pkgs/package_config/README.md

@@ -4,8 +4,7 @@ Support for working with **Package Resolution Configuration** files as described
 in this [DEP](https://github.com/lrhn/dep-pkgspec/blob/master/DEP-pkgspec.md), 
 under review [here] (https://github.com/dart-lang/dart_enhancement_proposals/issues/5).
 
-[![Build Status](https://travis-ci.org/dart-lang/package_config.svg)](https://travis-ci.org/dart-lang/package_config)
-
+[![Build Status](https://travis-ci.org/dart-lang/package_config.svg?branch=master)](https://travis-ci.org/dart-lang/package_config)
 
 ## Features and bugs