Fix for Django 1.7

@trbs, @end0, @owais, @kawa-marcin, 

I've applied the suggested patches to fix the call to `promote_joins` in Django 1.7.

I've also updated travis and tox scripts to include Django's new version.

However due to Django's new AppConfig boot sequence, the tests are not running on Django 1.7.

```pytb
py34-django1.7-postgres runtests: commands[1] | python runtests.py --settings tests.test_postgres
Creating test database for alias 'default'...
Traceback (most recent call last):
  File "runtests.py", line 44, in <module>
    runtests()
  File "runtests.py", line 40, in runtests
    sys.exit(runner.run_tests([]))
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/site-packages/django/test/runner.py", line 147, in run_tests
    old_config = self.setup_databases()
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/site-packages/django/test/runner.py", line 109, in setup_databases
    return setup_databases(self.verbosity, self.interactive, **kwargs)
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/site-packages/django/test/runner.py", line 299, in setup_databases
    serialize=connection.settings_dict.get("TEST_SERIALIZE", True),
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/site-packages/django/db/backends/creation.py", line 374, in create_test_db
    test_flush=True,
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/site-packages/django/core/management/__init__.py", line 93, in call_command
    app_name = get_commands()[name]
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/functools.py", line 428, in wrapper
    result = user_function(*args, **kwds)
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/site-packages/django/core/management/__init__.py", line 73, in get_commands
    for app_config in reversed(list(apps.get_app_configs())):
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/site-packages/django/apps/registry.py", line 137, in get_app_configs
    self.check_apps_ready()
  File "/Users/henrique/workspace/django-aggregate-if/.tox/py34-django1.7-postgres/lib/python3.4/site-packages/django/apps/registry.py", line 124, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.
```

I need to make all tests passing before releasing `0.5`. I plan to figure this out during the next week. Any help is very welcome.