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

app/models/valuator_group.rb

@@ -2,4 +2,6 @@ class ValuatorGroup < ActiveRecord::Base
   has_many :valuators
   has_many :valuator_group_assignments, dependent: :destroy, class_name: 'Budget::ValuatorGroupAssignment'
   has_many :investments, through: :valuator_group_assignments, class_name: 'Budget::Investment'
-end
+
+  validates :name, presence: true, uniqueness: true
+end