Do not vendor github.com/cloudflare/sike and github.com/henrydcase/nobs
in Go 1.12 and before, that does not work when using -mod=vendor.
Instead use the 'internal/' prefix as is done for golang.org/x/ packages
in Go 1.12.
Go 1.13 makes it possible to use the vendor directory, so use that.
Do not vendor github.com/cloudflare/sike and github.com/henrydcase/nobs in Go 1.12 and before, that does not work when using -mod=vendor. Instead use the 'internal/' prefix as is done for golang.org/x/ packages in Go 1.12.
Go 1.13 makes it possible to use the vendor directory, so use that.
Fixes #177