Fix uneeded double-quotes usage in Playlist specs

spec/models/playlist_spec.rb

@@ -4,11 +4,11 @@ describe Playlist do
   subject { playlist }
   let(:playlist) { Playlist.new :name => 'Electro' }
 
-  context "with valid attributes" do
+  context 'with valid attributes' do
     it { should be_valid }
   end
 
-  context "when name empty" do
+  context 'when name empty' do
     before do
       playlist.name = ''
     end