ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- selenium-webdriver (LoadError)

I am trying to run this below code, then I got code error:

ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- selenium-webdriver (LoadError)

I am not able to find what I am missing please help me with this

require "selenium-webdriver"# Firefox browser instantiationdriver = Selenium::WebDriver.for :firefox#Loading the assertselenium URLdriver.navigate.to "http://www.assertselenium.com"#Clicking on the Follow link present on the assertselenium home pageFollowButton  = driver.find_element(:link, "Follow")FollowButton.click#Typing the UserNameLoginButton = driver.find_element(:id, "user_login")LoginButton.send_keys "sampleuser77dff27"#Typing the Email-IdEmailId = driver.find_element(:id, "user_email")EmailId.send_keys "sampleuser7f7df27@gmail.com"#Clicking on the Submit ButtonSubmitButton = driver.find_element(:id, "wp-submit")SubmitButton.click#Asserting whether the registration success message is diaplyedSuccessMessage = driver.find_element(:css, "p.message")"Registration complete. Please check your e-mail.".eql? SuccessMessage.textputs "Successfully completed the user registration and validated the Success message"#Quitting the browserdriver.quit