Updated version numbers to 0.18.0 since we're skipping the 0.17.3 release

Summary: We're too close to the Aug 15th code-complete deadline to bother doing an intermediary release, so skipping 0.17.3 and going straight to 0.18.0 makes sense.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1610