Update README.md

Author: kciesielski

README.md

@@ -2,6 +2,7 @@ stringmask
 ==========
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.softwaremill.strignmask/stringmask_2.11/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.softwaremill.stringmask/stringmask_2.11)
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+[![Build Status](https://travis-ci.org/softwaremill/stringmask.svg?branch=master&style=flat)](https://travis-ci.org/softwaremill/stringmask)
 
 A micro-library for macro-based case class field masking in `.toString()`.
 Inspired by blog by Juan Pedro Moreno @47deg: http://www.47deg.com/blog/scala-macros-annotate-your-case-classes