Enable testing for Xception on Theano and CNTK

Author: taehoonlee

tests/applications_test.py

@@ -160,8 +160,6 @@ def test_vgg():
     _test_app_pooling(app, last_dim)
 
 
-@pytest.mark.skipif((backend.backend() != 'tensorflow'),
-                    reason='Xception supported only on TensorFlow')
 def test_xception():
     app = xception.Xception
     module = xception