Upgrade meek to 0.18
This is to fix a regression that broke meek in Tor Browser 4.5: #15872 (moved).
Here is the diff from 0.17. There are some other changes, but the only one that affects anything that goes into Tor Browser is the change to terminateprocess-buffer.go. https://gitweb.torproject.org/pluggable-transports/meek.git/diff/?id=0.18&id2=0.17