Update go 1.13

Author: jun06t

.travis.yml

@@ -1,6 +1,6 @@
 language: go
 go:
-- 1.12.5
+- 1.13.x
 env:
   global:
   - GO111MODULE=on

README.md

@@ -1,7 +1,7 @@
 go-iap
 ======
 
-![](https://img.shields.io/badge/golang-1.12-blue.svg?style=flat)
+![](https://img.shields.io/badge/golang-1.13-blue.svg?style=flat)
 [![Build Status](https://travis-ci.org/awa/go-iap.svg?branch=master)](https://travis-ci.org/awa/go-iap)
 [![codecov.io](https://codecov.io/github/awa/go-iap/coverage.svg?branch=master)](https://codecov.io/github/awa/go-iap?branch=master)