Minor fixes to minimatrix

Hi!

I saw (on Tindie) that you offer free hardware for improvements to the code, so I gave it a go.

- I fixed a build problem that arised from the Python script not being executable, probably just an oversight?
- I applied some simple ideas to the main.c firmware and shrunk the ELF by almost 3%, I hope that counts! :)

Please get back to me if this qualifies, and of course thanks for providing the firmware to your products!

/Emil