Unable to go get strfmt due to stringprep rename

[Phil Adams]

It seems that all of a sudden I can't install the strfmt module due to a nested dependency problem:

$ go get -u github.com/go-openapi/strfmt
go: downloading github.com/go-openapi/strfmt v0.20.0
go: github.com/go-openapi/strfmt upgrade => v0.20.0
go get: github.com/go-openapi/strfmt@v0.20.0 requires
	go.mongodb.org/mongo-driver@v1.4.3 requires
	github.com/xdg/stringprep@v0.0.0-20180714160509-73f8eece6fdc: invalid version: unknown revision 73f8eece6fdc

I think the solution to this is to upgrade the dependency on mongo-driver to v1.5.1.

[Phil Adams]

I've opened PR #82 to address this.