Support old versions of Moo

Moo only allows non-subroutine defaults in versions 1.001+, so switch
non-ref defaults to subroutines.

This should fix the 4 reported failures from the CPAN Testers project.
Don't attempt to generate LICENSE file

The @Basic Dist::Zilla bundle complains if there's already a LICENSE
file around, so work around it with @Filter.
Update documentation in module, not README.pod

Since README.pod gets generated from the module itself, edits should not
be made in README.pod directly.
Bump $VERSION to 0.007