Update balanced Gateway

Hey there!

Balanced has released an updated version of our API, and this moves the gateway to be in line with it. I may eventually submit another PR which streamlines a few HTTP calls, but this works well for now.

It's super fun implementing this gateway: ActiveMerchant was one of the first Ruby gems I ever used, way back in 2008. Time flies. :heart_eyes: :blush: :heart_eyes_cat: :sparkles: :boom: :tada: :confetti_ball: 

One thing I wasn't sure about: I nuked the unit tests. Is that okay? Since there are only 13 tests total, and it's easy to run just one, I actually developed it against just the remote tests:

```
$ time ruby -Ilib:test test/remote/gateways/remote_balanced_test.rb 
Run options: 

# Running tests:

Finished tests in 60.423246s, 0.2151 tests/s, 0.8771 assertions/s.            
13 tests, 53 assertions, 0 failures, 0 errors, 0 skips

ruby -v: ruby 2.0.0p353 (2013-11-22 revision 43784) [x86_64-linux]

real    1m1.814s
user    0m2.657s
sys     0m0.317s
```

The contributing guidelines didn't mention if they were actually required or not. If they are, I can re-do those too, but I'm not sure that mocking the responses is really worth it.

/cc @remear, @mjallday, @mahmoudimus.