Depend on recent-enough version of Plack:

Dancer2::Core::Runner::psgi_app method uses Plack::Builder to add
the Plack::Middleware::Head middleware. It uses the "wrap" method,
which was only introduced in version 1.0016.

Some don't update Plack unless there's a specific need, so we're
making sure it's provided.
reflect change
Merge branch 'fix/minimum-version-plack'