enforce dependency on six >= 1.5

[chantra]

Apparently the queue module made it somewhere around 1.5.0 and is required by sparts. in facebookarchive/augmented-traffic-control#74 the user is most likely having an old version on their system so it fail to start up. Given #90 I am not sure it is worth it, but should the version dependency be enforced in setup.py? Happy to send a diff your way if needs be,

[fmoo]

Ok, done. Do we need a (0.7.2) release to pin atc to?

[chantra]

no, not worth a new release. I just wanted to flag it as a dependency.

[zhangxiaang]

@chantra my 'six' version is lower than 1.5
then how can i update 'six' to 1.5 or Higher？
thank u very much!

[zhangxiaang]

..thanks!!!
i have resolved it by

sudo pip install six --upgrade