Validate ValuatorGroup#name presence & uniqueness

Why:

ValuatorGroup name should be unique and present to be able to identify
correctly each of them.

How:
  - Adding a presence & uniqueness validation at the model
  - Adding a sequenced value for name attribute at its factory
  - Adding missing model spec that covers validations

spec/models/valuator_group_spec.rb

@@ -0,0 +1,20 @@
+require 'rails_helper'
+
+describe ValuatorGroup do
+
+  describe 'Validations' do
+    it "should be valid" do
+      expect(build(:valuator_group)).to be_valid
+    end
+
+    it "should not be valid without a name" do
+      expect(build(:valuator_group, name: nil)).not_to be_valid
+    end
+
+    it "should not be valid with the same name as an existing one" do
+      create(:valuator_group, name: 'The Valuators')
+
+      expect(build(:valuator_group, name: 'The Valuators')).not_to be_valid
+    end
+  end
+end