Enable Rails/Validation Rubocop cop

Cop checks for the use of old-style attribute validation. Supports autocorrection which was run
to fix the files concerned

Closes #1414

app/models/vposition.rb

@@ -4,5 +4,5 @@ class Vposition < ActiveRecord::Base
   has_many :vchoices
   has_many :vdays, through: :vchoices
 
-  validates_presence_of :title, :vdays
+  validates :title, :vdays, presence: true
 end