[SECP256K1] Update Travis deprecated keywords

Summary:
The travis sudo key is deprecated and can be removed.
The matrix keyword is an obsolete alias for jobs.
This diffs update the .travis.yml file to accomodate the most recent
syntax.

Depends on D4856.

Test Plan:
Use the Travis CI Build Config Explorer to check the syntax:
https://config.travis-ci.com/explore

Before this diff, the "Parsed and validated config" shows warnings.
After this diff there is none.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4878