build: Use Go 1.12.9

This is a minor release of Go that does not include any changes that
affect Terraform's behavior.

This does include a fix for golang/go#31084 that could potentially affect
HCL arithmetic (via math/big) on aarch64, but we do not currently build
Terraform for aarch64 so it cannot have affected any previous releases.

.go-version

@@ -1 +1 @@
-1.12.8
+1.12.9

.travis.yml

@@ -4,7 +4,7 @@ services:
   - docker
 language: go
 go:
-- "1.12.8"
+- "1.12.9"
 
 # add TF_CONSUL_TEST=1 to run consul tests
 # they were causing timouts in travis