[SECP256K1] Fix Travis failures due to APT addon

Summary:
The Travis build sometimes fails with on of these symptoms:

• The cmake install works as expected, but the version selected by apt

is the one from the ubuntu repository (3.5) instead of the kitware one
(latest). This is the most common issue (see
https://travis-ci.org/Bitcoin-ABC/secp256k1/jobs/656567740).

• The signing key failed to import (see

https://travis-ci.org/Fabcien/secp256k1/jobs/657636912);

• The kitware repository cannot be fetched (see

https://travis-ci.org/Fabcien/secp256k1/jobs/657636911).

Since all my attempt to fix these issues failed, this diff avoids
completely the Travis APT addon and proceed the installation in a
script.

It is hard to tell if that fixes the issue since it doesn't happen all
the time, but my previous build had a lot of these errors
(https://travis-ci.org/Fabcien/secp256k1/builds/657636884) and the one
with this fix was green twice in a row
(https://travis-ci.org/Fabcien/secp256k1/builds/657657646 and
https://travis-ci.org/Fabcien/secp256k1/builds/657683100) so at least
it is not making things worse.

Depends on D5407 for the Travis run to succeed.

Test Plan:
Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/657657646).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5408