Relax js-base64 constraint to ^3.6

[holm]

js-base64 had a bug where mocha was added as a dependency, rather than a dev-dependency, see dankogai/js-base64#149.

This was fixed in 3.7.1, but cannot be used with this package, since it dependency is set to ~3.6.0. I think it would be better to relax to ^3.6.0, so minor updates can be included.

[frederickfogerty]

Hi @holm 👋 I've merged #305, which should fix this issue!

[holm]

Awesome, thank you for the quick response