[SECP256K1] Use the cmake version from Kitware PPA on Travis
Summary:
This PPA is well maintained by Kitware, and is often updated to the
latest version. This will allow to bump the version requirement past the
Travis version (currently 3.12).
Note: this fix an
issue with list(SORT) that would fail on empty lists
with cmake < 3.13. The minimum version will be bumped in a later diff to
enforce this fix.
Test Plan:
Run the Travis build, and check the cmake version in use is the one from
the PPA. See the build on repo (failure will be fixed in another diff):
https://travis-ci.org/Fabcien/secp256k1/builds/639851517
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5033