Updates version to v0.18.5

Summary: As per title. Update version strings to v0.18.5

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: jasonbcox, Mengerian, akb, #bitcoin_abc

Reviewed By: jasonbcox, Mengerian, akb, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2066