Configure dependabot to update all dependencies

Configure dependabot to update all dependencies

Previously, dependabot would only update gems mentioned in the gemfile,
and any dependencies of updated gems. This means that if the top-level
gem doesn't change, none of its dependencies were candidates.

So for a wrapper gem (like `dartsass-sprockets`) that doesn't need to
change often, the underlying dependencies (like `sprockets`) were often
becoming out-of-date.