Support v2.0 postman

[joolfe]

Not sure if version 2 of postman collection is used a lot but I think is not difficult to make it compatible.

[joolfe]

Reading this issue postmanlabs/postman-collection#635 seems like there are different just in -

• Auth helpers are an array instead of an object.
• Request URLs are always an object, while they were objects or strings in v2.0.