bump version to 0.18.4

Summary: prepared for next release as per release checklist

Test Plan: Read doc/release-process.md and check the checklist

Reviewers: jasonbcox, schancel, Fabien, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2031