Add missing six to requirements

requirements.in

@@ -2,3 +2,4 @@ backports.lzma; python_version < '3.3'
 click
 construct
 cryptography
+six

setup.py

@@ -25,6 +25,7 @@ requires = [
     'click',
     'construct',
     'cryptography',
+    'six',
 ]
 if sys.version_info[0] == 2:
     requires.append('backports.lzma')