update terraform wrapper

To update the latest GPG hashicorp keys

Change-Id: I392d167b4d698f06c3851a62002b551a8891d332
diff --git a/terraform b/terraform
index af18b2b..770c934 100755
--- a/terraform
+++ b/terraform
@@ -5,7 +5,7 @@
 
 update_terraform()
 {
-    export TFVERS=0.12.28
+    export TFVERS=0.12.31
     if [ ! -x $TOP/.bin/terraform_${TFVERS} ]
     then
         (