$ gem install bcrypt-ruby -v '3.0.0'
Temporarily enhancing PATH to include DevKit...
Building native extensions.  This could take a while...
ERROR:  Error installing bcrypt-ruby:
              ERROR: Failed to build gem native extension.

    current directory: D:/Programming/Apps/Ruby23-x64/lib/ruby/gems/2.3.0/gems/bcrypt-ruby-3.0.0/ext/mri
D:/Programming/Apps/Ruby23-x64/bin/ruby.exe -r ./siteconf20161002-12508-uikb1r.rb extconf.rb
creating Makefile

current directory: D:/Programming/Apps/Ruby23-x64/lib/ruby/gems/2.3.0/gems/bcrypt-ruby-3.0.0/ext/mri
make "DESTDIR=" clean

current directory: D:/Programming/Apps/Ruby23-x64/lib/ruby/gems/2.3.0/gems/bcrypt-ruby-3.0.0/ext/mri
make "DESTDIR="
Makefile:261: recipe for target 'bcrypt_ext-x64-mingw32.def' failed
process_begin: CreateProcess(NULL, echo generating bcrypt_ext-x64-mingw32.def, ...) failed.
make (e=2): The system cannot find the file specified.

make: *** [bcrypt_ext-x64-mingw32.def] Error 2

make failed, exit code 2

Gem files will remain installed in D:/Programming/Apps/Ruby23-x64/lib/ruby/gems/2.3.0/gems/bcrypt-ruby-3.0.0 for inspection.
Results logged to D:/Programming/Apps/Ruby23-x64/lib/ruby/gems/2.3.0/extensions/x64-mingw32/2.3.0/bcrypt-ruby-3.0.0/gem_make.out