You install DevKit from same site.
DevKit-mingw64-32-4.7.2-2D13D224-1151.sfx.exe and put
After execute DevKit.exe file, you make
devkit directory and throw PATH to
C:¥devkit>ruby dk.rb init [INFO] found RubyInstaller v2.1.6 at C:/Ruby21 Initialization complete! Please review and modify the auto-generated 'config.yml' file to ensure it contains the root directories to all of the installed Rubies you want enhanced by the DevKit. C:¥devkit>
C:¥> gem install rspec C:¥> gem install selenium-webdriver C:¥> gem install capybara C:¥> gem install pry
You can install which one.
And put where you thrown PATH.