Support Django 2.2 and stop RemovedInDjango30Warning

Taken from:

commit a2bcd795ed68d0b5882e853e731624f572cbb28e
Author: Adam Johnson <me@adamj.eu>
Date:   Sun Apr 28 23:08:54 2019 +0100

    Support Django 2.2 and stop RemovedInDjango30Warning (#13)

    * Add HISTORY note and update README
    * Add 2.2 to tox testing grid
    * Update `classifiers` in `setup.py`

without the related documentation/build changes.

jsonfield/fields.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 import json
 
+import django
 from django.core.exceptions import ValidationError
 from django.conf import settings
 from django.db import models
@@ -80,11 +81,16 @@ class JSONField(models.Field):
             return 'long'
         return 'text'
 
-    def from_db_value(self, value, expression, connection, context):
-        if value is None:
-            return None
-        return json.loads(value, **self.decoder_kwargs)
-
+    if django.VERSION > (2, 0):
+        def from_db_value(self, value, expression, connection):
+            if value is None:
+                return None
+            return   json.loads(value, **self.decoder_kwargs)
+    else:
+        def from_db_value(self, value, expression, connection, context):
+            if value is None:
+                return None
+            return json.loads(value, **self.decoder_kwargs)
 
     def get_db_prep_value(self, value, connection=None, prepared=None):
         return self.get_prep_value(value)