internal: add go1.9 to module
internal: add go1.9 to module

Go 1.13 is quite insistent that this be there.

Change-Id: I8ad3e0d36fb45ae5942237a8ab9a58ed329a802b
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/43690
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@google.com>
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>

googleapis/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client