Remove "build status" badge (drone went away)

README.md

diff --git a/README.md b/README.md
index 0998e6a..5d0c31d 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# set [![GoDoc](https://godoc.org/github.com/xtgo/set?status.png)](https://godoc.org/github.com/xtgo/set) [![Build Status](https://drone.io/github.com/xtgo/set/status.png)](https://drone.io/github.com/xtgo/set/latest) [![Coverage Status](https://coveralls.io/repos/xtgo/set/badge.svg?branch=master&service=github)](https://coveralls.io/github/xtgo/set?branch=master)
+# set [![GoDoc](https://godoc.org/github.com/xtgo/set?status.png)](https://godoc.org/github.com/xtgo/set) [![Coverage Status](https://coveralls.io/repos/xtgo/set/badge.svg?branch=master&service=github)](https://coveralls.io/github/xtgo/set?branch=master)
 
 Package set provides type-safe, polymorphic mathematical set operations that operate on any sort.Interface implementation.