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>

util/bot/go/bootstrap.py

diff --git a/util/bot/go/bootstrap.py b/util/bot/go/bootstrap.py
index 166ef3b..1125619 100755
--- a/util/bot/go/bootstrap.py
+++ b/util/bot/go/bootstrap.py
@@ -45,7 +45,7 @@
 EXE_SFX = '.exe' if sys.platform == 'win32' else ''
 
 # Pinned version of Go toolset to download.
-TOOLSET_VERSION = 'go1.4'
+TOOLSET_VERSION = 'go1.5.1'
 
 # Platform dependent portion of a download URL. See http://golang.org/dl/.
 TOOLSET_VARIANTS = {