Upgrade to SLF4J 1.7.6 and Logback 1.1.1

Another janitorial pull request.

* [[qos.ch-announce] Release of SLF4J version 1.7.6](http://mailman.qos.ch/pipermail/announce/2014/000125.html)
* Logback 1.1.1: http://jira.qos.ch/browse/LOGBACK-942 and http://jira.qos.ch/browse/LOGBACK-943