[SECP256K1] Use the cmake version from Kitware PPA on Travis
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.
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):
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5033