along the way drop django1.9 and 1.10

Latitude and longitude are documented in the wrong order for in_bbox filter.

Also adds Django 1.11 to Travis, closes #132.

Fixes https://github.com/djangonauts/django-rest-framework-gis/issues/120.

The Django docs gives the following warning for `JSONField` (should be valid for `HStoreField` as well):

> If you give the field a default, ensure it’s a callable such as dict (for an empty default) or a callable that returns a dict (such as a function). Incorrectly using default={} creates a mutable default that is shared between all instances of JSONField.

Fixes #107.

Closes #95

The change introduced in the previous commit was optimized
in order to avoid losing the performance gains introduced
in the latest versions (0.9.3, 0.9.4).

By default GeoFeatureModelSerializer will use the attributes of a
model instance (except id, geometry field and bbox field) to fill
the "properties" member of the resulting GeoJSON object.

This commit separates this functionality in a few methods,
which can be overriden by subclasses of GeoFeatureModelSerialzier.
This way users will be able to override the default behaviour in
order to supply their own source for the properties member.

Makes GeoModelSerializer obsolete.

Created sections.