I upgraded my environment from Ruby 2.0.0 to 2.2.3.
I also upgraded (overwrite) DevKit, and re-run ruby dk.rb install.
I removed Gemfile.lock and ran bundle
I spent an entire hour fixing this, just now.
Well what I did was I followed some advice online to do a
gem install bcrypt --platform=ruby
And then, it worked in irb.
irb(main):001:0> require 'bcrypt'
=> true
Later when I had to do a bundle install, for some odd reason rails installed another bcrypt and the error was back. So I had two folders in my gem root.
bcrypt-3.1.10
bcrypt-3.1.10-x64-mingw32
So, since the first folder was the one that got built with the devkit, and it works. I deleted the contents of the second folder and copied the contents of the first into it.
Seems to be working as I'm writing this.