2-0-stable branch has a dependency on 'spree_core (~> 2.1.0)'

Hi Guys,

A commit 12 days ago changed the 2-0-stable branch's dependency on `spree_core` to ~> 2.1.0

I think this need to be fixed?

https://github.com/spree/spree_print_invoice/commit/fce9789525b2e7d34773fc809874bb73cf4303f5#diff-26c1a21d5645e61fa28b4eb1ffef373dR17

Thanks,

David