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

README.md

 The Micro Python project
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-  <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"/>
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 This is the Micro Python project, which aims to put an implementation