Unbreak 'make test'

The 'vendor' directory was removed because it keeps getting outdated
compared to go.mod.

Otherwise 'make test' fails with:
  $ make test
  go test -v -cover ./...
  go: inconsistent vendoring in /path/to/briandowns/spinner:
	golang.org/x/[email protected]: is
	    explicitly required in go.mod, but not marked as explicit in
	    vendor/modules.txt
	golang.org/x/[email protected]: is marked
	    as explicit in vendor/modules.txt, but not explicitly
	    required in go.mod

	To ignore the vendor directory, use -mod=readonly or -mod=mod.
	To sync the vendor directory, run:
		go mod vendor
  make: *** [Makefile:12: test] Error 1

Fallout from 12e6c29