Fix admin typo & timezone issue. 

1) The first change set is to fix an obvious typo which leads to exception when visiting /admin/djstripe/invoice/ .

2) Timezone issue could be reproduced by changing the timezone of the local system to utf+n timezone (n>0, eg: utf+8), then the logic covered by tests/test_managers.py:test_started_during_has_records will failed. Install pytz will ensure the test timezone neutral.