Fix BitPay Helper to use form_fields for generating invoice id
Fix BitPay Notification status
Implement BitPayNotification#acknowledge to request invoice
BitPay Notification#gross should return price, not btcPrice
parse posData as JSON string, and handle JSON ParseErrors
Fix incorrect notification definition which should allow for options to be passed
Fix status mapping
Refactor Paymill gateway

* Removes duplication
* Extracts complex method to class

Closes #824.
Merge pull request #908 from Shopify/fix_bitpay

Fix BitPay implementation
Update changelog to include BitPay fixes
Bump version to 1.42.0
Better rake task for gem release
Merge pull request #914 from Shopify/gem_release

Better rake task for gem release
Merge pull request #913 from Shopify/bump_v

Bump version to 1.42.0
Update date on 1.42.0 release
Bump for version 1.42.1
New updated public_cert
Version bump
Require private key to build gem
Update RELEASING file