Enables RSpec/ExampleWording and fixes all issues

Both avoiding 'should' and repiting 'it' on the tests description
improves reading them and also makes all descriptions consistent.

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

spec/models/follow_spec.rb

@@ -4,21 +4,21 @@ describe Follow do
 
   let(:follow) { build(:follow, :followed_proposal) }
 
-  it "should be valid" do
+  it "is valid" do
     expect(follow).to be_valid
   end
 
-  it "should not be valid without a user_id" do
+  it "is not valid without a user_id" do
     follow.user_id = nil
     expect(follow).to_not be_valid
   end
 
-  it "should not be valid without a followable_id" do
+  it "is not valid without a followable_id" do
     follow.followable_id = nil
     expect(follow).to_not be_valid
   end
 
-  it "should not be valid without a followable_type" do
+  it "is not valid without a followable_type" do
     follow.followable_type = nil
     expect(follow).to_not be_valid
   end