[TRAVIS] Install cmake version 3.16

Summary: I forgot to update the Travis vesion in D7073.

Test Plan:
Checked the checksum and link are OK, not tested the Travis build but
this is trivial enough and obviously necessary.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7101