Fixed dependency on django-mssql v1.6

Depend on current production django-mssql
Fixed tests when run with django-mssql 1.6, tests were failing because
pagination wasn't working correctly with django-mssql v1.6
Moved mark_tests_as_expected_failure to runtests

Previously this call was causing client's tests to fail.
Since this piece of code is only about testing django-sqlserver itself
it should be in tests/runtests.py