Upgrade to Logback 1.1.8

[philwebb]

Logback 1.1.8 has now been released but we'll need to change DefaultLogbackConfiguration as SizeBasedTriggeringPolicy.setMaxFile now takes a FileSize object.

Thanks to @ceki for the early heads-up on this.

[philwebb]

I know we discounted the back-compatible approach as too complex but I'd quite like to reconsider that. We've had people need to stay on older versions in the past and I'd like us to be compatible with any 1.1.x version if possible.