tests: don't build test/libfixmath on Travis CI

Very often the build unexplainably hangs.
Merge pull request #1648 from Kijewski/no-test-libfixmath

tests: don't build test/libfixmath on Travis CI
sys:ringbuffer: use memcpy in ringbuffer_get
sys:ringbuffer: static inline ringbuffer_init
SQUASH ME: bugs