Install all the pack dependencies in the pylint script, but re-use single shared global virtual environment for all the pack.
Install all the pack dependencies in the pylint script, but re-use single shared
global virtual environment for all the pack.

This should speed things up and prevent Travis from timing out. It should work
fine as long as different packs don't have conflicting dependencies.

digideskio/st2contrib,psychopenguin/st2contrib,pearsontechnology/st2contrib,lmEshoo/st2contrib,digideskio/st2contrib,psychopenguin/st2contrib,StackStorm/st2contrib,pidah/st2contrib,meirwah/st2contrib,pearsontechnology/st2contrib,pidah/st2contrib,pearsontechnology/st2contrib,armab/st2contrib,tonybaloney/st2contrib,tonybaloney/st2contrib,meirwah/st2contrib,pidah/st2contrib,lmEshoo/st2contrib,StackStorm/st2contrib,StackStorm/st2contrib,armab/st2contrib,armab/st2contrib,tonybaloney/st2contrib,pearsontechnology/st2contrib