Make GitHub logo/image a JPEG so it's smaller.

Author: dpgeorge

README.md

@@ -1,7 +1,7 @@
 The Micro Python project
 ========================
 <p align="center">
-  <img src="https://raw2.github.com/micropython/micropython/master/logo/upython-with-micro.png" alt="MicroPython Logo"/>
+  <img src="https://raw2.github.com/micropython/micropython/master/logo/upython-with-micro.jpg" alt="MicroPython Logo"/>
 </p>
 
 This is the Micro Python project, which aims to put an implementation