Fixed Django 1.7+ deprecation warning for requires_model_validation

See https://docs.djangoproject.com/en/1.8/howto/custom-management-commands/#django.core.management.BaseCommand.requires_system_checks

Also removed a rather low value test which wasn't worth rewriting.

compressor/management/commands/compress.py

@@ -60,8 +60,6 @@ class Command(NoArgsCommand):
             dest="engine"),
     )
 
-    requires_model_validation = False
-
     def get_loaders(self):
         if django.VERSION < (1, 8):
             from django.template.loader import template_source_loaders
@@ -302,3 +300,9 @@ class Command(NoArgsCommand):
                     "Offline compression is disabled. Set "
                     "COMPRESS_OFFLINE or use the --force to override.")
         self.compress(sys.stdout, **options)
+
+
+if django.VERSION < (1, 7):
+    Command.requires_model_validation = False
+else:
+    Command.requires_system_checks = False

compressor/tests/test_offline.py

@@ -201,9 +201,6 @@ class OfflineCompressBasicTestCase(OfflineTestCaseMixin, TestCase):
         for engine in self.engines:
             self._test_deleting_manifest_does_not_affect_rendering(engine)
 
-    def test_requires_model_validation(self):
-        self.assertFalse(CompressCommand.requires_model_validation)
-
     def test_get_loaders(self):
         TEMPLATE_LOADERS = (
             ('django.template.loaders.cached.Loader', (