[Rails] Omniauth provider error in devise

I'm trying to add omniauth functionality in my app. For authentication
I'm using devise

This is what I'm doing

# GemFile

gem 'omniauth'
gem 'omniauth-github', '~> 1.1.2'

# Devise.rb
   config.omniauth :github, 'ID', 'Secret key'

# User.rb

devise :database_authenticatable, :registerable, :confirmable,
          :recoverable, :rememberable, :trackable, :validatable,
          :omniauthable, :omniauth_providers => [:github]

   def self.from_omniauth(auth)
     where(provider: auth.provider, uid: auth.id).first_or_create do
       user.provider = auth.provider
       user.uid = auth.uid
       user.email = auth.info.email
       user.password = Devise.friendly_token[0,20]
     end
   end

Whenever I do http://localhost/login it shows me following error message [0]

Where I'm making mistake, please correct me.

[0] : http://awesomescreenshot.com/0d9492h900