link build badge to master branch

and change it to svg

Author: a0viedo

readme.markdown

@@ -4,7 +4,7 @@ tap-producing test harness for node and browsers
 
 [![browser support](https://ci.testling.com/substack/tape.png)](http://ci.testling.com/substack/tape)
 
-[![build status](https://secure.travis-ci.org/substack/tape.png)](http://travis-ci.org/substack/tape)
+[![build status](https://secure.travis-ci.org/substack/tape.svg?branch=master)](http://travis-ci.org/substack/tape)
 
 ![tape](http://substack.net/images/tape_drive.png)