[SECP256K1] Fix the kitware PPA timeouts issues on Travis

Summary:
This is a follow-up for D5408. After several builds, I got one timeout
from the kitware PPA that caused a build failure.

This diff is installing cmake from the prebuilt binaries. It uses the
first supported stable version directly downloaded from cmake.org.

Hopefully this will make the cmake installation more robust on Travis.
It makes no difference in the build duration.

Test Plan:
Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/657837937).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5428