added more build status links

Author: a-brandt

README.md

@@ -3,7 +3,11 @@
 
 # arangodb-java-driver
 
-[![Build Status](https://travis-ci.org/arangodb/arangodb-java-driver.svg?branch=3.0)](https://travis-ci.org/arangodb/arangodb-java-driver)
+2.7: [![Build Status](https://secure.travis-ci.org/arangodb/arangodb-java-driver.svg?branch=2.7)](https://travis-ci.org/arangodb/arangodb-java-driver) 
+3.0: [![Build Status](https://secure.travis-ci.org/arangodb/arangodb-java-driver.svg?branch=3.0)](https://travis-ci.org/arangodb/arangodb-java-driver) 
+3.1: [![Build Status](https://secure.travis-ci.org/arangodb/arangodb-java-driver.svg?branch=3.1)](https://travis-ci.org/arangodb/arangodb-java-driver) 
+
+master: [![Build Status](https://secure.travis-ci.org/arangodb/arangodb-java-driver.svg?branch=master)](https://travis-ci.org/arangodb/arangodb-java-driver)
 
 This library is a Java driver for ArangoDB.