README: show logo on Travis badge

Author: tarleb

README.md

@@ -2,7 +2,7 @@
 
 [![Travis build status][Travis badge]](https://travis-ci.org/pandoc/lua-filters)
 
-[Travis badge]: https://img.shields.io/travis/pandoc/lua-filters/master.svg?label=travis+build
+[Travis badge]: https://img.shields.io/travis/pandoc/lua-filters.svg?logo=travis
 
 This repository collects Lua filters for pandoc.