Add `json` to list of gems required for test server

Encountered the error below when setting up the dev environment. Resolved by building the Xcode license and installing the json gem, which wasn't installed by the bundler.

• sudo gem install json, after running
• sudo xcodebuild -license

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require': no such file to load -- json (LoadError)
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `require'
    from ./server.rb:3
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `require'
    from /Users/me/Exp/miso/dataset/test/config.ru:4
    from /Library/Ruby/Gems/1.8/gems/rack-1.4.1/lib/rack/builder.rb:51:in `instance_eval'
    from /Library/Ruby/Gems/1.8/gems/rack-1.4.1/lib/rack/builder.rb:51:in `initialize'
    from /Users/me/Exp/miso/dataset/test/config.ru:1:in `new'
    from /Users/me/Exp/miso/dataset/test/config.ru:1