Added Travis badge

Author: langecode

README.md

@@ -10,6 +10,7 @@ Module is considered production ready.
 
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.openapitools.swagger/swagger-maven-plugin/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.openapitools.swagger/swagger-maven-plugin/)
 [![Javadoc](https://javadoc-emblem.rhcloud.com/doc/io.openapitools.swagger/swagger-maven-plugin/badge.svg)](https://www.javadoc.io/doc/io.openapitools.swagger/swagger-maven-plugin)
+[![Build status](https://travis-ci.org/openapi-tools/swagger-maven-plugin.svg?branch=master)](https://travis-ci.org/openapi-tools/swagger-maven-plugin)
 
 # Usage