Allow six to bump

requirements-dev.txt

diff --git a/requirements-dev.txt b/requirements-dev.txt
index a7dac81..97343d5 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -7,4 +7,3 @@
 pre-commit
 pylint<1.4
 pytest
-six==1.9.0

setup.py

diff --git a/setup.py b/setup.py
index f09e6d5..7a66dcf 100644
--- a/setup.py
+++ b/setup.py
@@ -32,7 +32,7 @@
         'autopep8>=1.1',
         'pyyaml',
         'simplejson',
-        'six==1.9.0',
+        'six',
     ],
     entry_points={
         'console_scripts': [