all: add go.mod

Commands run:
	go mod init
	go mod edit -go=1.11
	go mod tidy
	go list -m all
	go test ./...

Updates golang/go#30228

Change-Id: I3d7e4f59a98822691177973ec2d7a55a6216b78c
Reviewed-on: https://go-review.googlesource.com/c/tour/+/167181
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go.mod

@@ -0,0 +1,5 @@
+module golang.org/x/tour
+
+go 1.11
+
+require golang.org/x/tools v0.0.0-20190312164927-7b79afddac43

go.sum

@@ -0,0 +1,7 @@
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/net v0.0.0-20190311183353-d8887717615a h1:oWX7TPOiFAMXLq8o0ikBYfCJVlRHBcsciT5bXOrH628=
+golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/tools v0.0.0-20190312164927-7b79afddac43 h1:TIYuaOnWereVgpmGdM420uV4ykKf3r5m++5s628rZdE=
+golang.org/x/tools v0.0.0-20190312164927-7b79afddac43/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=