Refactor spec_helper requirement with rspec config

spec/models/playlist_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 describe Playlist do
   subject         { playlist }
   let(:playlist)  { FactoryGirl.build(:playlist) }

spec/models/sound_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 describe Sound do
   subject     { sound }
   let(:sound) { FactoryGirl.build(:sound) }

spec/models/track_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 describe Track do
   subject     { track }
   let(:track) { FactoryGirl.build(:track) }

spec/models/user_spec.rb

@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 describe User do
   subject(:user) { FactoryGirl.build(:user) }