Enable RSpec/NotToNot cop and fix all issues

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

spec/shared/models/document_validations.rb

@@ -14,13 +14,13 @@ shared_examples "document validations" do |documentable_factory|
   it "is not valid without a title" do
     document.title = nil
 
-    expect(document).to_not be_valid
+    expect(document).not_to be_valid
   end
 
   it "is not valid without an attachment" do
     document.attachment = nil
 
-    expect(document).to_not be_valid
+    expect(document).not_to be_valid
   end
 
   it "is valid for all accepted content types" do
@@ -35,28 +35,28 @@ shared_examples "document validations" do |documentable_factory|
   it "is not valid for attachments larger than documentable max_file_size definition" do
     document.stub(:attachment_file_size).and_return(maxfilesize.megabytes + 1.byte)
 
-    expect(document).to_not be_valid
+    expect(document).not_to be_valid
     expect(document.errors[:attachment]).to include "must be in between 0 Bytes and #{maxfilesize} MB"
   end
 
   it "is not valid without a user_id" do
     document.user_id = nil
 
-    expect(document).to_not be_valid
+    expect(document).not_to be_valid
   end
 
   it "is not valid without a documentable_id" do
     document.save
     document.documentable_id = nil
 
-    expect(document).to_not be_valid
+    expect(document).not_to be_valid
   end
 
   it "is not valid without a documentable_type" do
     document.save
     document.documentable_type = nil
 
-    expect(document).to_not be_valid
+    expect(document).not_to be_valid
   end
 
 end