chore(internal/kokoro): use 1.15 for regen (#649)

diff --git a/internal/kokoro/synth/synth.sh b/internal/kokoro/synth/synth.sh
index c40e117..e0c9d03 100755
--- a/internal/kokoro/synth/synth.sh
+++ b/internal/kokoro/synth/synth.sh
@@ -7,6 +7,14 @@
 set -e -x
 
 cd $(dirname $0)/../../..
+
+# Install Go 1.15
+rm -rf /usr/local/go
+curl -o /tmp/go.tgz https://dl.google.com/go/go1.15.1.linux-amd64.tar.gz &&
+    tar -C /usr/local -xzf /tmp/go.tgz &&
+    rm /tmp/go.tgz &&
+    export PATH=$PATH:/usr/local/go/bin
+
 go version
 go env
 
diff --git a/internal/kokoro/vet.sh b/internal/kokoro/vet.sh
index 0aaa23f..1245f1b 100755
--- a/internal/kokoro/vet.sh
+++ b/internal/kokoro/vet.sh
@@ -9,8 +9,8 @@
 
 # Only run the linter on go1.13, since it needs type aliases (and we only care
 # about its output once).
-if [[ `go version` != *"go1.13"* ]]; then
-    exit 0
+if [[ $(go version) != *"go1.15"* ]]; then
+  exit 0
 fi
 
 go install \