Add Golang 1.12 to CI

.travis.yml

@@ -8,6 +8,7 @@ language: go
 go_import_path: "miniflux.app"
 go:
   - "1.11"
+  - "1.12"
 before_install:
   - npm install -g jshint
   - go get -u golang.org/x/lint/golint