update hacking to 0.6

allow hacking 0.6 to enforce new rules in tempest, especially the
missing license header one.

Change-Id: I787cbc942989b50c57cbc22e4bf1e4956aa64df6
diff --git a/test-requirements.txt b/test-requirements.txt
index 2185997..693daff 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -2,7 +2,6 @@
 pep8==1.4.5
 pyflakes==0.7.2
 flake8==2.0
-hacking>=0.5.3,<0.6
+hacking>=0.5.6,<0.7
 # needed for doc build
 sphinx>=1.1.2
-