Update version to 0.18.7

Summary: As per title

Test Plan:
Build with make and cmake
Read the files

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Mengerian

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox, Mengerian

Subscribers: Mengerian, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2287