Adding needed json gem

[D3vil0p3r]

@digininja needed gem json to prevent the error:

CeWL 6.1 (Max Length) Robin Wood (robin@digi.ninja) (https://digi.ninja/)

Error: json gem not installed
	use: "gem install json" to install the required gem

[digininja]

What OS are you running on? JSON is one of the default gems and as far as I know always has been. https://stdgems.org/

[D3vil0p3r]

What OS are you running on? JSON is one of the default gems and as far as I know always has been. https://stdgems.org/

Arch Linux

[digininja]

I've just checked and others are saying that they get json by default on Arch so I'm going to say no to this one but if anyone else reports a problem I'll come back and add it in.