Update ebuild to use EAPI 5

Updated btsync version to latest release 1.3.94.

Hi,

I'm not really sure what I'm doing with Github... But hopefully this is "right way" to do a pull request.
I've sort-of tested this ebuild on Gentoo... However I can't really test the Openrc service file (as I haven't use Openrc for years). The systemd service units all appear to work OK.

Is there any reason to leave in the older btsync version ebuilds? Personally I can only see this introducing sync'ing compatibility issues between OS's...

Robert