Allow to skip test_aci_rest if no xmljson is installed

There is no such package in Fedora so building the Fedora package fails. See
e970237a2f_ffe3b87d/rpmbuild.log

Introduced by e970237a2f

test/units/modules/network/aci/test_aci_rest.py

@@ -25,9 +25,12 @@ from ansible.modules.network.aci.aci_rest import aci_response
 
 from nose.plugins.skip import SkipTest
 
-from lxml import etree
-if sys.version_info >= (2, 7):
+try:
+    from lxml import etree
+    if sys.version_info >= (2, 7):
         from xmljson import cobra
+except ImportError:
+    raise SkipTest("aci Ansible modules require the lxml and xmljson Python libraries")
 
 
 class AciRest(unittest.TestCase):