Enable RSpec/LetBeforeExamples cop & fix issues

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

spec/models/budget/group_spec.rb

@@ -2,10 +2,10 @@ require 'rails_helper'
 
 describe Budget::Group do
 
-  it_behaves_like "sluggable"
-
   let(:budget) { create(:budget) }
 
+  it_behaves_like "sluggable"
+
   describe "name" do
     before do
       create(:budget_group, budget: budget, name: 'object name')

spec/models/budget/heading_spec.rb

@@ -2,11 +2,11 @@ require 'rails_helper'
 
 describe Budget::Heading do
 
-  it_behaves_like "sluggable"
-
   let(:budget) { create(:budget) }
   let(:group) { create(:budget_group, budget: budget) }
 
+  it_behaves_like "sluggable"
+
   describe "name" do
     before do
       create(:budget_heading, group: group, name: 'object name')