Fix warning adding if test name is longer then 256 chars

According to PyTest changes: pytest-dev/pytest@fd48cd5

Fixes #120

Author: Parviz Khavari

pytest_reportportal/service.py

@@ -10,6 +10,7 @@
 from _pytest.nodes import File, Item
 from _pytest.python import Class, Function, Instance, Module
 from _pytest.unittest import TestCaseFunction, UnitTestCase
+from _pytest.warning_types import PytestWarning
 from reportportal_client import ReportPortalServiceAsync
 from six import with_metaclass
 from six.moves import queue
@@ -454,9 +455,10 @@ def _get_item_name(test_item):
         if len(name) > 256:
             name = name[:256]
             test_item.warn(
-                'C1',
-                'Test node ID was truncated to "{}" because of name size '
-                'constrains on reportportal'.format(name)
+                PytestWarning(
+                    'Test node ID was truncated to "{}" because of name size '
+                    'constrains on reportportal'.format(name)
+                )
             )
         return name