Fix strange problem manifested with py 2.7.12

Python 2.7.12 + tox + Postgres make a lot of tests fail. I do not
understand why. Running tests in a different order
(by explicitly listing tests directories) somehow fixes the problem.
Also don't know why. Let it stay this way for some time.

tox.ini

@@ -42,7 +42,7 @@ commands = {[mysql]commands}
 [postgresql]
 commands =
     psql -c 'create database sqlobject_test;' -U postgres
-    py.test --cov=sqlobject -D postgres://postgres:@localhost/sqlobject_test
+    py.test --cov=sqlobject -D postgres://postgres:@localhost/sqlobject_test tests include/tests inheritance/tests versioning/test
     psql -c 'drop database sqlobject_test;' -U postgres
 
 [testenv:py26-postgres]