Update README.md

Author: AdmiralObvious

README.md

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 Flask-SimpleLDAP
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-[![Build Status](https://travis-ci.org/admiralobvious/flask-simpleldap.png?branch=master)](https://travis-ci.org/admiralobvious/flask-simpleldap)
+[![Build Status](https://travis-ci.org/admiralobvious/flask-simpleldap.svg?branch=master)](https://travis-ci.org/admiralobvious/flask-simpleldap)
 
 Flask-SimpleLDAP provides LDAP authentication for Flask.