Added index on issues.state

This field is queried when filtering issues and due to the lack of an
index would end up triggering a sequence scan.

db/migrate/20151109134526_add_issues_state_index.rb

@@ -0,0 +1,5 @@
+class AddIssuesStateIndex < ActiveRecord::Migration
+  def change
+    add_index :issues, :state
+  end
+end