Fixed case of content-type header in nginx config

http* urls pointing to metadata files failed due to vagrant doing an case-sensitive check for the content-type header to be application/json to identify if the url points to a metadata file.

See: https://github.com/mitchellh/vagrant/blob/v1.6.3/lib/vagrant/action/builtin/box_add.rb#L488

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Thank you so much for this great tutorial @hollodotme, my own vagrant cloud is now up and running!