Update Django versions after bugfix release

Fix empty pk detection in HyperlinkRelatedField.get_url

Update Django security releases 1.9.3 and 1.8.10

add rest-framework-generic-relations link to docs

Add `/en` to Heroku guidelines link

[change] using safer calling way of  "@api_view" in docs/tutorial/5-relationships-and-hyperlinked-apis.md

remove comma

This implementation allows detection of empty values that are non-nullable, allowing the field to return None values for such cases

"@api_view" usage error.

Fixes incorrect references to URLPathVersioning

updated tutorial for django 1.9

Docs: Fix repetitive word in the tutorial

Fix typo in relations docs

Create issue and pull request templates

Closes #3952: Remove old django checks from tests and compat

Closes #3937. Support callable as the value of `initial` for any `serializer.Field`

Just like regular Django fields, the example adopted uses a `datetiume.date.today` to
illustrate the functionality.

Remove skipping of tests for Django<1.8. Remove several functions from compat.py not needed for Django>=1.8

fix typo

fix typo

fix typo

fix typo

Add an usage example on how to pass a callable to the `Field.initial` in the docs.

Drop EOL Django 1.7

Fixed _force_text_recursive typo

Add missing space in serializer error message