app/appengine: update to Go 1.9 so we can use std context

Go 1.9 is the latest offered on App Engine, which at least is new
enough to get us type aliases.

Fixes golang/go#26115

Change-Id: I4de3062cf2f7df70216536a05b7150324c1a5e91
Reviewed-on: https://go-review.googlesource.com/121418
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>

Author: bradfitz

app/appengine/app.yaml

@@ -1,5 +1,5 @@
 runtime: go
-api_version: go1.8
+api_version: go1.9
 
 handlers:
 - url: /static