Remove hard dependency on version 1.2.10 for log4net

The version of log4net is fixed to 1.2.10. Hence one is unable to
use the log4net extension in a project using a higher version of
log4net. I have removed this restriction.
I don't fully understand the significacne of the PublicKeyToken and not so sure of the repercussions here. However, the issue I was trying to resolve was using LogExtensions in my current project. However, it isn't possible to do so because the latter is using a more recent version of log4net that is in LogExtensions. Hence, I enabled this by allowing for higher versions of log4net to be used with LogExtensions. There might well be a better, cleaner way but I don't know it. So, if you could outline how you would have done it, maybe I can try and implement your suggestion.