Rspec test in rails error - ruby-on-rails

Question

I am newbie in rspec test.
I have a file /spec/models/class_spec.rb. The content of the file is
require 'rails_helper'
RSpec.describe Class, :type => :model do
#pending "add some examples to (or delete) #{__FILE__}"
before(:each) do
#my_class=Factory.create(:class, params)
end
end
I also have a /spec/rails_helper file. When I run rspec in command line
command : rspec spec/models/class_spec.rb
I gets the following error
/usr/local/rvm/gems/ruby-2.3.1/gems/airbrake-ruby-1.4.4/lib/airbrake-ruby.rb:102:in `configure': the 'default' notifier was already configured (Airbrake::Error)
from /home/akhil/test-prjct/ver3/this_prjct/config/initializers/errbit.rb:1:in `<top (required)>'
from /usr/local/rvm/gems/ruby-2.3.1/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `load'
from /usr/local/rvm/gems/ruby-2.3.1/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `block in load'
from /usr/local/rvm/gems/ruby-2.3.1/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:240:in `load_dependency'
from /usr/local/rvm/gems/ruby-2.3.1/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:268:in `load'
Can someone tell me about issue and also give a link to a tutorial for learning to write test codes using rspec. Thanks in advance
My gem list is
group :development, :test do
gem "rails-erd"
gem 'letter_opener'
gem 'spring'
gem 'faker'
gem 'pry-rails'
gem 'pry-byebug'
gem 'rspec-rails'
gem 'factory_girl_rails'
end
errbit.rb file has following content
Airbrake.configure do |config|
config.host = 'https://192.168.2.143:3000'
config.project_id = -1
config.project_key = '76068df1155420d2658i35a75o95gk26'
config.environment = Rails.env
config.ignore_environments = %w(development test)
end