[SECP256k1] Add the CMake/Ninja build to Travis

Summary:
This adds a CMake build script to be run by Travis.

Since some configuration switches are not part of the CMake options,
only the relevant combinations are used for the CMake builds.

Depends on D4855.

Test Plan: Run the travis build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4856