Fix min version mentioned in README file

Author: Rajat Goel

README.md

@@ -1,6 +1,6 @@
 # Dropbox SDK for Go [UNOFFICIAL] [![GoDoc](https://godoc.org/github.com/dropbox/dropbox-sdk-go-unofficial/dropbox?status.svg)](https://godoc.org/github.com/dropbox/dropbox-sdk-go-unofficial/dropbox) [![Build Status](https://travis-ci.org/dropbox/dropbox-sdk-go-unofficial.svg?branch=master)](https://travis-ci.org/dropbox/dropbox-sdk-go-unofficial) [![Go Report Card](https://goreportcard.com/badge/github.com/dropbox/dropbox-sdk-go-unofficial)](https://goreportcard.com/report/github.com/dropbox/dropbox-sdk-go-unofficial)
 
-An **UNOFFICIAL** Go SDK for integrating with the Dropbox API v2. Tested with Go 1.5+
+An **UNOFFICIAL** Go SDK for integrating with the Dropbox API v2. Tested with Go 1.11+
 
 :warning: WARNING: This SDK is **NOT yet official**. What does this mean?