[SECP256K1] Fix some travis issues on macOS

Summary:

• The Homebrew addon is not compatible with versioned package installation, so do it manually: https://travis-ci.org/github/Fabcien/secp256k1/jobs/731184075
• The valgrind test times out on macOS. As per https://reviews.bitcoinabc.org/D7633?id=23949#inline-43274 the test is run twice, with autotools and cmake, which isn't really necessary. Running it only with autotools will help avoiding the timeout.

Test Plan:
Run the Travis build.
Actually failing:
https://travis-ci.org/github/Fabcien/secp256k1/builds/731184003

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7661