Ah, I see. I think base.rb changed recently and now loads the Rails environment immediately. This also loads bundler which overrides require, and since mysql2 is not in Discourse’s bundle, it can’t be loaded.
Fixed here:
https://github.com/Elberet/discourse/commit/1cc7db23707a70889dd135c0cde32ff1850013e5
Grab the update from my fork (or just edit it into your local copy of the script – it’s three lines…) or wait until it’s merged into the core repo. 
CC @neil and @gerhard since the phpbb3 and, I’d assume, all other importers will likely have the same issue.