Update django template and caching documentation links (django-compre…

…ssor#1110)

Updated a link pointing to the docs for the now deprecated django 2.1 to the docs for the dev edition of django.

Also changed the link to django templates to point to the /stable/ endpoint.

docs/usage.txt

@@ -131,7 +131,7 @@ exception. If DEBUG is ``False`` these files will be silently stripped.
     Django's `caching documentation`_).
 
 .. _memcached: http://memcached.org/
-.. _caching documentation: https://docs.djangoproject.com/en/2.1/topics/cache/#memcached
+.. _caching documentation: https://docs.djangoproject.com/en/stable/topics/cache/#memcached
 
 
 .. _offline_compression:
@@ -180,7 +180,7 @@ in a file called ``manifest.json`` using the :attr:`configured storage
 <django.conf.settings.COMPRESS_STORAGE>` to be able to be transferred from your development
 computer to the server easily.
 
-.. _TEMPLATE_LOADERS: http://docs.djangoproject.com/en/dev/ref/settings/#template-loaders
+.. _TEMPLATE_LOADERS: http://docs.djangoproject.com/en/stable/ref/settings/#template-loaders
 
 .. _signals: