travis: cython3 -> cython, add ca-certificates.
travis: cython3 -> cython, add ca-certificates.

twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc