Update assets as per spree/spree#4050
Added maxipago gateway

Fixes #127
Fix BalancedGateway bug to create profile when active_marchant has changed the response of Store function for Balanced

Fixes #129
Fixes #130
Fixes #131
Switch back to using Firefox tests, as Poltergeist seems to be having issues
Include expiration in tests where Stripe is supposed to through invalid error messages for other things
Refactor specs + fixes

- Cleanup + Refactor spec syntax (specs tend to be reused so why not
keep them well styled)
- Fixed Braintree a bit that had Test::Unit syntax in it, also
described pending tests
- Fixed SecurePayAU Gateway that had it active_merchant gateway
misspelled
- Fixed UsaEpay spec
- Add base classes for 11 gateways
- Updated read me and Versionfile
- Bumped dependencies
Remove braintree dependency

https://github.com/spree/spree_gateway/pull/138/files#r10213250
Add braintree as a development dependency
Depend on Spree 2.3.0.beta
Bump to 2.3.0.beta
Correct email + homepage + files for gem
Removed savon dependency

Fixes #139
Use new preferences setter in AuthorizeNetCIM class
Correct arguments for BraintreeGateway's capture method
Remove BraintreeGateway#preferences

This is no longer needed with the recent preferences changes on Spree's master branch
Use new preferences references within braintree_gateway_spec to appease the testing gods
Remove ancient pending block for authorize+capture payment flow within BraintreeGateway's tests
Ignored options on BraintreeGateway#capture should be optional
Use correct options when calling capture in BraintreeGateway#capture tests
Braintree Credit test won't work because Braintree doesn't provide a way (yet) to settle the transactions