Unfluff the CI.

With transactional fixtures enabled, the session records would end up in
@_current_transaction_records, and at the end of the transaction,
methods would be called on them that would trigger method_missing and
trigger attribute methods to be generated.

However, at this point the sessions table would not exist, and the
columns were not cached, so an exception would be raised because we
can't find the columns to generate attribute methods for.

Not sure exactly why this didn't crop up before but there have been
changes to the schema cache code and perhaps that means that column data
that was cached previously at that point is now uncached.

activerecord/test/cases/session_store/session_test.rb

@@ -5,7 +5,7 @@
 module ActiveRecord
   class SessionStore
     class SessionTest < ActiveRecord::TestCase
-      self.use_transactional_fixtures = false unless supports_savepoints? && ActiveRecord::Base.connection.supports_ddl_transactions?
+      self.use_transactional_fixtures = false
 
       def setup
         super