git2consul can't handle null value

[eyalzek]

Coming from the following line:
https://github.com/Cimpress-MCP/git2consul/blob/v0.12.13/lib/consul/index.js#L152

If I have a json file where one of the values is a null value, git2consul willl fail parsing the json and will put the entire structure as a string instead.