Add new endpoints from the May 2019 update

[vthn]

The documentation lists several methods that are not actually available in the current version. e.g. Get Hierarchy (chargebee.customer.hierarchy) will result in an error.

I'm sure adding these is in the pipeline and I'm submitting this issue more as a reminder for myself, so I can remove my manual:

[ 
 "hierarchy",
 "GET",
 "/customers",
 "/hierarchy",
 true
]

when the update comes out / issue is closed.

That said, I can submit a PR with some of the added endpoints if you'd like.

[cb-goutham]

This has been fixed in v2.5.1