Make caffeine compatible with older browsers

[netpok]

Many older browser does not support let and const language elements so it is required to change them to var.

[mikebronner]

Thank you for this! I will take a look over the next week or so. (My computer underwent surgery over the last few days, so I'm playing catchup and reinstalling things.)

[netpok]

Thanks, I updated the test cases and fixtures to reflect these changes, it's all green locally.