README.md: fix travis-ci badge
Apache Commons Pool
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-[![Build Status](https://travis-ci.org/apache/commons-pool2.svg?branch=master)](https://travis-ci.org/apache/commons-pool2)
+[![Build Status](https://travis-ci.org/apache/commons-pool.svg?branch=master)](https://travis-ci.org/apache/commons-pool)
[![Maven Central](https://maven-badges.herokuapp.com/maven-central/org.apache.commons/commons-pool2/badge.svg)](https://maven-badges.herokuapp.com/maven-central/org.apache.commons/commons-pool2/)
Apache Commons Object Pooling Library
