Making the gem compatible with activemodel ~4

I'm updating an application which uses your API from rails 3 to rails 4.
I was stumbling on a error when I tried to use Bitstamp.ticker

This error:
```ruby
NoMethodError: undefined method `vwap=' for #<Bitstamp::Ticker:0x007f885d4df070>
```

In this PR I'm sending my commits which solved this error. I ran your tests against activemodel/activesupport ~> 4 and activemodel/activesupport ~> 3.1 and they all passed.

Best regards.