doc: Add travis badge

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

README.md

 Libav
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+[![Build Status](https://travis-ci.org/libav/libav.svg)](https://travis-ci.org/libav/libav)
+
 Libav is a collection of libraries and tools to process multimedia content
 such as audio, video, subtitles and related metadata.