Remove tests which duplicate db structure

These tests repeat the implementation and provide limited value.

Also, it is a goal to remove the dependency on the shoulda gem.

spec/models/version_spec.rb

@@ -6,13 +6,6 @@ describe PaperTrail::Version, type: :model do
   end
 
   describe "Attributes" do
-    it { is_expected.to have_db_column(:item_type).of_type(:string) }
-    it { is_expected.to have_db_column(:item_id).of_type(:integer) }
-    it { is_expected.to have_db_column(:event).of_type(:string) }
-    it { is_expected.to have_db_column(:whodunnit).of_type(:string) }
-    it { is_expected.to have_db_column(:object).of_type(:text) }
-    it { is_expected.to have_db_column(:created_at).of_type(:datetime) }
-
     describe "object_changes column", versioning: true do
       let(:widget) { Widget.create!(name: "Dashboard") }
       let(:value) { widget.versions.last.object_changes }
@@ -37,10 +30,6 @@ describe PaperTrail::Version, type: :model do
     end
   end
 
-  describe "Indexes" do
-    it { is_expected.to have_db_index([:item_type, :item_id]) }
-  end
-
   describe "Methods" do
     describe "Instance" do
       subject { PaperTrail::Version.new }

spec/rails_helper.rb

@@ -10,7 +10,6 @@ require "spec_helper"
 require File.expand_path("../../test/dummy/config/environment", __FILE__)
 require "rspec/rails"
 require "paper_trail/frameworks/rspec"
-require "shoulda/matchers"
 require "ffaker"
 require "timecop"