ci: move to 1.12.x for the latest Go version (#57)

Author: myitcv

.travis.yml

@@ -5,7 +5,7 @@ os:
   - osx
 go:
   - "1.11.x"
-  - "1.12beta2"
+  - "1.12.x"
 env:
   - GO111MODULE=on
 go_import_path: github.com/rogpeppe/go-internal