add build status to README (#18)

Author: isopov

README.md

@@ -1,4 +1,8 @@
-[![GoDoc](https://godoc.org/github.com/joomcode/errorx?status.svg)](https://godoc.org/github.com/joomcode/errorx) [![Report Card](https://goreportcard.com/badge/github.com/joomcode/errorx)](https://goreportcard.com/report/github.com/joomcode/errorx) [![gocover.io](https://gocover.io/_badge/github.com/joomcode/errorx)](https://gocover.io/github.com/joomcode/errorx) [![Mentioned in Awesome Go](https://awesome.re/mentioned-badge.svg)](https://github.com/avelino/awesome-go#miscellaneous)
+[![Build Status](https://travis-ci.com/joomcode/errorx.svg?branch=master)](https://travis-ci.com/joomcode/errorx)
+[![GoDoc](https://godoc.org/github.com/joomcode/errorx?status.svg)](https://godoc.org/github.com/joomcode/errorx)
+[![Report Card](https://goreportcard.com/badge/github.com/joomcode/errorx)](https://goreportcard.com/report/github.com/joomcode/errorx)
+[![gocover.io](https://gocover.io/_badge/github.com/joomcode/errorx)](https://gocover.io/github.com/joomcode/errorx)
+[![Mentioned in Awesome Go](https://awesome.re/mentioned-badge.svg)](https://github.com/avelino/awesome-go#miscellaneous)
 
 ## Highlights