cmd/gopherbot: update deps to pull in latest go-github
cmd/gopherbot: update deps to pull in latest go-github

Maintner needs the IsPullRequest property of GitHubIssue.

Change-Id: I6a5f457a3dea552c9418c8e6b627fb7918f26ba9
Reviewed-on: https://go-review.googlesource.com/80375
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build