Bump copyright year to 2020

Summary: See title.

Test Plan:

ninja
bitcoind --version
bitcoin-qt --version

bitcoin-cli and bitcoin-tx do not have copyright years in --version|--help

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5698