Require json gem for specs.

Faraday middleware is not compatible with Ruby 2.0's JSON library.

Produces many errors such as:
```
  1) BreweryDB::Resource#get an OK request returns the data
     Failure/Error: get('breweries', name: 'Rogue Ales').data
     Faraday::Error::ParsingError:
       uninitialized constant JSON::Parser
     # ./lib/brewery_db/request.rb:20:in `response_body'
     # ./lib/brewery_db/request.rb:10:in `response'
     # ./lib/brewery_db/resource.rb:13:in `get'
     # ./spec/brewery_db/resource_spec.rb:10:in `ok'
     # ./spec/brewery_db/resource_spec.rb:26:in `block (4 levels) in <top (required)>'
```