[release-branch.go1.13-security] go1.13.15

Change-Id: I4922f24774168f1c991b486276257a1ed18d34b6
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/814547
Reviewed-by: Katie Hockman <katiehockman@google.com>

VERSION

diff --git a/VERSION b/VERSION
index be38e0e..1cad8be 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-go1.13.14
\ No newline at end of file
+go1.13.15
\ No newline at end of file