Merge pull request #972 from terraform-providers/go_upgrade_1_12_3

Update travis.yml to use go 1.12.3

Author: parrneet

.travis.yml

@@ -4,7 +4,7 @@ services:
 - docker
 language: go
 go:
-- "1.11.x"
+- "1.12.3"
 
 install:
 # This script is used by the Travis build to install a cookie for