Update chainparams for 0.18.5 release

Summary: Not enough blocks on testnet since the fork to bother updating testnet params.

Test Plan: N/A (IBD will come later for sanity)

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2093