Use mutli_json gem instead of json.

[nosborn]

gemspec is missing an explicit dependency on the json gem. Switch to multi_json just because it's already required for fog.

[nosborn]

Just to clarify, the gemspec is currently broken:

$ git clone https://github.com/alphagov/vcloud-box-spinner.git temp
...
$ cd temp
$ bundle install --standalone
...
$ bundle exec bin/vcloud-box-spinner 
./bin/../lib/provisioner/cli.rb:1:in `require': no such file to load -- json (LoadError)
    from ./bin/../lib/provisioner/cli.rb:1
    from bin/vcloud-box-spinner:4:in `require'
    from bin/vcloud-box-spinner:4