Support for Logback 1.1

[felixbarny]

Currently, we require Logback 1.2 as the Encoder interface has changed.

https://github.com/qos-ch/logback/blob/v_1.1.0/logback-core/src/main/java/ch/qos/logback/core/encoder/Encoder.java
https://github.com/qos-ch/logback/blob/v_1.2.0/logback-core/src/main/java/ch/qos/logback/core/encoder/Encoder.java

It should be possible to support older versions by implementing the method the 1.1 versions defied, without annotating them with @Override.