Remove six from requirements

It was vendorised in 75c430635b46de090165bcc5140050fe0f1dd56d

requirements.txt

@@ -1,6 +1,5 @@
 docopt==0.6.1
 PyYAML==3.10
 requests==2.2.1
-six>=1.3.0
 texttable==0.8.1
 websocket-client==0.11.0