xxd invoked incorrectly

[ThomasWaldmann]

https://travis-ci.org/ThomasWaldmann/py-esp32-ulp/builds/355488052?utm_source=email&utm_medium=notification

[ThomasWaldmann]

Hmm, not really incorrectly, the xxd on travis is just too old and does not know -e...

[ThomasWaldmann]

I'll do the fix in my current PR to get it working...