Upgrade development requirements.
Upgrade development requirements.

* coverage to 4.2
* psutil to 4.4.2
* pytest to 3.0.3
* pytest-cov to 2.4.0
* pytest-django to 3.0.0 (to get some more pytest 3 compat)
* pytest-xdist to 1.15.0

mozilla/addons-server,diox/olympia,aviarypl/mozilla-l10n-addons-server,bqbn/addons-server,bqbn/addons-server,mstriemer/addons-server,harikishen/addons-server,mstriemer/addons-server,psiinon/addons-server,kumar303/olympia,psiinon/addons-server,atiqueahmedziad/addons-server,eviljeff/olympia,wagnerand/olympia,Revanth47/addons-server,kumar303/addons-server,kumar303/olympia,atiqueahmedziad/addons-server,harry-7/addons-server,mozilla/olympia,lavish205/olympia,mstriemer/addons-server,bqbn/addons-server,wagnerand/olympia,mozilla/addons-server,mozilla/olympia,harikishen/addons-server,wagnerand/addons-server,mstriemer/olympia,mozilla/olympia,atiqueahmedziad/addons-server,psiinon/addons-server,mozilla/addons-server,mstriemer/addons-server,psiinon/addons-server,mstriemer/olympia,tsl143/addons-server,mozilla/olympia,harry-7/addons-server,lavish205/olympia,wagnerand/addons-server,mstriemer/olympia,wagnerand/olympia,bqbn/addons-server,kumar303/addons-server,harikishen/addons-server,harikishen/addons-server,eviljeff/olympia,Revanth47/addons-server,kumar303/addons-server,Revanth47/addons-server,harry-7/addons-server,eviljeff/olympia,diox/olympia,wagnerand/addons-server,mozilla/addons-server,aviarypl/mozilla-l10n-addons-server,aviarypl/mozilla-l10n-addons-server,tsl143/addons-server,diox/olympia,wagnerand/addons-server,aviarypl/mozilla-l10n-addons-server,tsl143/addons-server,lavish205/olympia,kumar303/olympia,eviljeff/olympia,kumar303/olympia,mstriemer/olympia,diox/olympia,lavish205/olympia,atiqueahmedziad/addons-server,tsl143/addons-server,wagnerand/olympia,harry-7/addons-server,Revanth47/addons-server,kumar303/addons-server