Six is a runtime requirement not a test req

Six needs to be installed when python-jenkins is installed. By default
test-requirements are only installed when running tests. Six was listed
as a test-requirement so it was not installed when pip installing
python-jenkins. Make six a regular requirement so that it is installed
properly every time.

Change-Id: I2b7cb7e967b20ee0e06fe4c665f08c5932e524f2