Fix EnvHelpers.provisioners helper for vagrant 1.7

I set out to add support for the new vagrant 1.7 chef_zero provisioner and noticed vagrant 1.7 changed the meaning of config.vm.provisioners.name (basically renamed to config.vm.provisioners.type).

This is a fix to the EnvHelper module to account for both pre-1.7 and post-1.7 behavior.
@sethvargo merging this anyway since 1.7.x is already in the wild