Enable RSpec/DescribeClass rubocop rspec cop & fix issues

Manually fixes all issues & "stashed" false positives at .rubocop_todo.yml

Read about the cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DescribeClass

spec/models/poll/booth_assignment_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-describe :booth_assignment do
+describe Poll::BoothAssignment do
   let(:poll){create(:poll)}
   let(:booth){create(:poll_booth)}
   let(:booth1){create(:poll_booth)}
@@ -26,4 +26,4 @@ describe :booth_assignment do
 
     expect(Poll::Shift.all.count).to eq(0)
   end
-end
+end