Fix broken link

Replaced the current link with the one provided by
https://travis-ci.org/jgraph/drawio, if you click on the
badge there.

README.md

@@ -1,4 +1,4 @@
-[![Build Status](https://travis-ci.org/jgraph/drawio.png?branch=master)](https://travis-ci.org/jgraph/draw.io)
+[![Build Status](https://travis-ci.org/jgraph/drawio.svg?branch=master)](https://travis-ci.org/jgraph/drawio)
 
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