README: use SVG for the Travis badge as well

The new Coveralls URI points to SVG so let us stick with a single format
for all badges.

README.md

 # Ensembl Core API
 
-[![Build Status](https://travis-ci.org/Ensembl/ensembl.png?branch=master)][travis]
+[![Build Status](https://travis-ci.org/Ensembl/ensembl.svg?branch=master)][travis]
 [![Coverage Status](https://coveralls.io/repos/github/Ensembl/ensembl/badge.svg?branch=master)][coveralls]
 
 [travis]: https://travis-ci.org/Ensembl/ensembl