Update .go-version to 1.12.9 (#323)

Both the plugin SDK and Terraform itself are using Go 1.12.9.
This updates use to that version as well as updating Travis
to test on 1.12.x and 1.13.x

https://github.com/hashicorp/terraform/blob/master/.go-version
https://github.com/hashicorp/terraform-plugin-sdk/blob/master/.go-version

.go-version

@@ -1 +1 @@
-1.11.5
+1.12.9

.travis.yml

@@ -4,7 +4,8 @@ services:
 - docker
 language: go
 go:
-  - "1.11.x"
+  - "1.12.x"
+  - "1.13.x"
 
 install:
 # This script is used by the Travis build to install a cookie for