Store DataMapper::MigrationRunner#migrations in a class variable

Fixes an issue where migrations don't get run at
all in dm-rails rake tasks.

Further explanation can be found in the related
ticket on LH. This is weird. Maybe it has to do
something with the way rails loads the rake tasks
and invokes our stuff. No idea ...

[#1214 state:resolved]

lib/dm-migrations/migration_runner.rb

@@ -76,7 +76,7 @@ def migrate_down!(level = nil)
     end
 
     def migrations
-      @migrations ||= []
+      @@migrations ||= []
     end
 
   end