Added drone.io build status to README.md

README.md

-Crystal [![Build Status](https://travis-ci.org/manastech/crystal.png)](https://travis-ci.org/manastech/crystal)
+Crystal [![Build Status](https://travis-ci.org/manastech/crystal.png)](https://travis-ci.org/manastech/crystal) [![Build Status](https://drone.io/github.com/manastech/crystal/status.png)](https://drone.io/github.com/manastech/crystal/latest)
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 Crystal is a programming language with the following goals: