Fix merit_actions migration template (#357)

if `add_index` is outside of the `def self.change` method, it's executed
right away, and:

    Caused by:
    ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  relation "merit_actions" does not exist

lib/merit/generators/active_record/templates/create_merit_actions.erb

@@ -11,7 +11,7 @@ class CreateMeritActions < ActiveRecord::Migration<%= migration_version %>
       t.boolean :processed, default: false
       t.timestamps null: false
     end
-  end
 
-  add_index :merit_actions, :processed
+    add_index :merit_actions, :processed
+  end
 end