use svg logo

Author: nschloe

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-  <img alt="tikzplotlib" src="https://nschloe.github.io/tikzplotlib/logo-tikzplotlib.png" width="60%">
+  <img alt="tikzplotlib" src="https://nschloe.github.io/tikzplotlib/logo-tikzplotlib.svg" width="60%">
 </p>
 
 [![CircleCI](https://img.shields.io/circleci/project/github/nschloe/tikzplotlib/master.svg?style=flat-square)](https://circleci.com/gh/nschloe/tikzplotlib/tree/master)