[SECP256K1] Move the autotools Travis build to it's own script
This will make it easier to follow when introducing the cmake build into
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