Upgrade dependencies

[jamesl33]

The dependencies are currently quite out-of-date, which means users of go-mockdns are - in some cases - required to use replace directives to pin versions to avoid security issues (e.g. in x/net).

$ go get -u ./...
go: upgraded github.com/miekg/dns v1.1.25 => v1.1.57
go: upgraded golang.org/x/net v0.0.0-20190923162816-aa69164e4478 => v0.18.0
go: upgraded golang.org/x/tools v0.0.0-20190907020128-2ca718005c18 => v0.15.0

$ go mod tidy

$ go test ./... -count=1
ok      github.com/foxcpp/go-mockdns    0.004s

[foxcpp]

Thanks!