Refactor spec_helper requirement with rspec config

spec/features/home_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 feature 'Home page' do
   include UserIntegrationHelpers
 

spec/features/playlists_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 feature 'Playlists' do
   include UserIntegrationHelpers
 

spec/features/tracks_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 feature 'Tracks' do
   include UserIntegrationHelpers
 

spec/features/user_sign_in_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 feature 'User sign in' do
   scenario 'redirects to the home page when not signed in' do
     visit root_path

spec/features/user_sign_up_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 feature 'User sign up' do
   let(:user) { FactoryGirl.build(:user) }