rubocop: fix various style issues.

db/migrate/20161229001201_drop_pool_versions.rb

@@ -1,9 +1,9 @@
 class DropPoolVersions < ActiveRecord::Migration[4.2]
   def up
-  	drop_table :pool_versions
+    drop_table :pool_versions
   end
 
   def down
-  	raise NotImplementedError
+    raise NotImplementedError
   end
 end