Build with Go 1.12.13

This is the latest 1.12 minor release at the time of writing. We are not
yet upgrading to Go 1.13 because it ends support for MacOS 10.10 and
earlier (Yosemite) and for versions of FreeBSD prior to 11.2, and so we
need to make that switch with care to properly phase those out as
supported platforms in Terraform too.

.go-version

@@ -1 +1 @@
-1.12.9
+1.12.13

.travis.yml

@@ -4,7 +4,7 @@ services:
   - docker
 language: go
 go:
-- "1.12.9"
+- "1.12.13"
 
 # add TF_CONSUL_TEST=1 to run consul tests
 # they were causing timouts in travis