Remove excess_inventory from purchase_cost calculation (fixes #26)

[joefarrington]

Fixes #26 , removing excess_inventory from the calculation of the purchase cost in the Newsvendor environment.

This change makes the or-gym version consistent with the original implementation by Balaji et al, in which the corresponding line is:

purchase_cost = self.gamma ** self.l * c * buys

(see https://github.com/awslabs/or-rl-benchmarks/blob/master/News%20Vendor/src/news_vendor_environment.py, their self.l is the lead time, c is the purchase cost and buys is the order quantity).