resolve @sliverc review method of using optional django-filter.

See #466 (comment)

Author: n2ygk

rest_framework_json_api/filters/__init__.py

@@ -1,2 +1,6 @@
+import pkgutil
 from .sort import JSONAPIOrderingFilter  # noqa: F401
-from .filter import JSONAPIDjangoFilter  # noqa: F401
+# If django-filter is not installed, no-op.
+if pkgutil.find_loader('django_filters') is not None:
+    from .filter import JSONAPIDjangoFilter  # noqa: F401
+del pkgutil