Skipped a couple of test with unsupported postgres features

tests/test_connection.py

@@ -1520,6 +1520,7 @@ class PasswordLeakTestCase(ConnectingTestCase):
 
 class SignalTestCase(ConnectingTestCase):
     @slow
+    @skip_before_postgres(8, 2)
     def test_bug_551_returning(self):
         # Raise an exception trying to decode 'id'
         self._test_bug_551(query="""

tests/test_types_extras.py

@@ -1085,6 +1085,7 @@ class JsonTestCase(ConnectingTestCase):
         self.assert_(s.endswith("'"))
 
     @skip_if_no_json_module
+    @skip_before_postgres(8, 2)
     def test_scs(self):
         cnn_on = self.connect(options="-c standard_conforming_strings=on")
         cur_on = cnn_on.cursor()