Add BridgePay gateway

http://www.bridgepaynetwork.com/

BridgePay says that to be able to update a transaction the 'Capture'
TransType should not be used since that locks the record. There may need
to be a nightly process enabled on the backend to process the batch of
transactions for the day.

Closes #1120.
Fix PxPay invalid credential response handling
No assert_raise_with_message in older rails
Rename Transnational to Network Merchants
Merge pull request #1124 from Shopify/fix_pxpay_invalid_creds

Fix PxPay invalid credential response handling
Change Travis notifications to go to payments team
Merge pull request #1125 from Shopify/nmi_rename

Rename Transnational to Network Merchants
BridgePay: Switch method of success detection.

In test the 'Message' field returned APROVAL but in production it was
'AUTH/TKT  02992G'. It appears that check the RespMSG field will be a
better choice.