(Bug) "Cache mode is on" output breaks json formatter

[keithlayne]

Not sure really whether to describe this as a bug or a feature request. Found this line:

https://github.com/Shopify/erb-lint/blob/8c629ebdf48d4eb00a92e1b407fffc0857ca6c44/lib/erb_lint/cache.rb#L12

This seems like out-of-band info that at worst is just` noise, at best probably belongs in stderr.