Replace deprecated stub call with double

Newer versions of rspec-mocks have deprecated `stub` in favor of `double`, resulting in warning messages when using `stub_db` in apps that use couch_potato (see https://github.com/rspec/rspec-mocks/pull/233). 

After simply swapping out `stub` for `double` in `stub_db`, all specs still pass and everything appears fine, and the warnings are gone.