added travis CI badge

README.md

@@ -1 +1 @@
-# errors [![GoDoc](https://godoc.org/github.com/pkg/errors?status.png)](http://godoc.org/github.com/pkg/errors)
+# errors [![Travis-CI](https://travis-ci.org/pkg/errors.svg)](https://travis-ci.org/pkg/errors) [![GoDoc](https://godoc.org/github.com/pkg/errors?status.png)](http://godoc.org/github.com/pkg/errors)