gddo: update gae-service-proxy for go111 GAE runtime

+ The go111 runtime requires google.golang.org/appengine to be
  updated to v1.6.5 in order for legacy APIs to continue to work
+ Small changes need to be made to the main package so that there
  is a proper main function

Steps run to update vendored dependencies:

  go get -d google.golang.org/appengine@v1.6.5
  go mod tidy
  go mod vendor

Change-Id: Ibfec53f00e8ba4874c36478607f68b0e652e09f6
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/216298
Reviewed-by: Bryan C. Mills <bcmills@google.com>