README: Use the SVG Travis badge

Author: kevinji

README.md

@@ -1,6 +1,6 @@
 # Tmux Resurrect
 
-[![Build Status](https://travis-ci.org/tmux-plugins/tmux-resurrect.png?branch=master)](https://travis-ci.org/tmux-plugins/tmux-resurrect)
+[![Build Status](https://travis-ci.org/tmux-plugins/tmux-resurrect.svg?branch=master)](https://travis-ci.org/tmux-plugins/tmux-resurrect)
 
 Restore `tmux` environment after system restart.