[SECP256K1] Move the autotools Travis build to it's own script

Summary:
This will make it easier to follow when introducing the cmake build into
Travis.

There is no change in behavior.

Test Plan: Run the script, run the travis build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4855