README: move the build badge to the badge list (#555)

Signed-off-by: zhulongcheng <zhulongcheng.me@gmail.com>

README.md

-# TSDB [![Build Status](https://travis-ci.org/prometheus/tsdb.svg?branch=master)](https://travis-ci.org/prometheus/tsdb)
+# TSDB 
 
+[![Build Status](https://travis-ci.org/prometheus/tsdb.svg?branch=master)](https://travis-ci.org/prometheus/tsdb)
 [![GoDoc](https://godoc.org/github.com/prometheus/tsdb?status.svg)](https://godoc.org/github.com/prometheus/tsdb)
 [![Go Report Card](https://goreportcard.com/badge/github.com/prometheus/tsdb)](https://goreportcard.com/report/github.com/prometheus/tsdb)