Add json to list of gems required for test server

[protobi]

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