Add JSON require to solusvm.rb

When SolusVM master is returning JSON feedback, SolusVM gem throws following error:

```
/home/antun/.gem/ruby/2.1.0/gems/solusvm-2.0.0.beta3/lib/solusvm/base.rb:68:in `parse_response': uninitialized constant SolusVM::Base::JSON (NameError)
```

Once JSON is loaded, all requests are parsed properly.