Bump six requirement to 1.10.0

The environment that Travis-CI sets up is somehow broken with regards
to the requirements for six.  The exact message we get and running
selftests/run is:

   pkg_resources.ContextualVersionConflict: (six 1.9.0
   (/home/travis/virtualenv/python3.4_with_system_site_packages/lib/python3.4/site-packages),
   Requirement.parse('six>=1.10.0'), {'Fabric3'})

Let's bump the requirements to 1.10.0, to try to help Travis-CI.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

requirements-travis.txt

@@ -13,7 +13,7 @@ mock==1.2.0; python_version <= '2.7'
 aexpect==1.0.0
 psutil==3.1.1
 # six is a stevedore depedency, but we also use it directly
-six==1.9.0
+six==1.10.0
 # stevedore for loading "new style" plugins
 stevedore==1.20.0
 lxml>=3.4.4

requirements.txt

@@ -4,6 +4,6 @@ pyliblzma>=0.5.3
 # REST client (avocado.core.restclient)
 requests>=1.2.3
 # six is a stevedore depedency, but we also use it directly
-six>=1.9.0
+six>=1.10.0
 # stevedore for loading "new style" plugins
 stevedore>=0.14