Only run public_data:pull if there are no pending migrations

Oh this was a fun little dev environment bug: I ran `public_data:pull`
on my laptop before migrating my database, so the `items` table pulled
as the latest production version, which included the migrations, but
they hadn't been marked as "run" yet.

So Rails was still telling me I needed to run them, but the migrations
themselves were crashing, with stuff like "there's already a column
with this name!"

This change ensures that `public_data:pull` won't run until migrations
are done, to prevent silly accidents like that.

lib/tasks/public_data.rake

@@ -63,7 +63,7 @@ namespace :public_data do
 	end
 
 	desc "Pull and import the latest public data from production (dev only)"
-	task :pull => :environment do
+	task :pull => ["db:abort_if_pending_migrations", :environment] do
 		unless Rails.env.development?
 			raise "Can only pull public data in development mode! This helps us " +
 				"ensure we won't overwrite the production database accidentally."