Add missing 'six' dependency to nailgun

Due to the missing entry for 'six' in requirements.txt for nailgun
in some cases old version of six was used (from other packages) which
was causes issues with missing features in older version of six.

Now this dependency has been added.

Change-Id: Ib44dbd02e6d775ca269460d836d9a2c354513e3b
Closes-Bug: #1366728
Merge "Add missing 'six' dependency to nailgun"