Update to Go 1.9 on the bots.

Change-Id: I692424f05f543c98a994a444f0303ea0bda7c14f
Reviewed-on: https://boringssl-review.googlesource.com/19725
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

util/bot/go/bootstrap.py

@@ -45,7 +45,7 @@ WORKSPACE = os.path.join(ROOT, 'go')
 EXE_SFX = '.exe' if sys.platform == 'win32' else ''
 
 # Pinned version of Go toolset to download.
-TOOLSET_VERSION = 'go1.8.3'
+TOOLSET_VERSION = 'go1.9'
 
 # Platform dependent portion of a download URL. See http://golang.org/dl/.
 TOOLSET_VARIANTS = {