Backward compatibility

[AlejandroHerr]

Hi!

I'm using LazyHttpKernel in companion with URL Map, but had problemas since my shared hosting doesn't use the 5.4 version of php. It could be nice to remove the type hint 'callable' to make it more compatible, since it's the only feature of php 5.4 used in the code.

Regards and thanks for your work!