Move the bots to Go 1.5.1.

Nothing in particular, but probably good to match the version of Go on our
workstations.

Change-Id: I5f4828299d56d25cd8c0dadfa91e9f18212a178d
Reviewed-on: https://boringssl-review.googlesource.com/6060
Reviewed-by: Adam Langley <agl@google.com>
1 file changed