Upgrade to logback 1.1.6

Release notes:

http://logback.qos.ch/news.html

pom.xml

@@ -301,7 +301,7 @@
     <dependency>
       <groupId>ch.qos.logback</groupId>
       <artifactId>logback-classic</artifactId>
-      <version>1.1.5</version>
+      <version>1.1.6</version>
     </dependency>
     <dependency>
       <groupId>junit</groupId>