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.
The PublicKeyTokens are different so :(

This is the single biggest issue I have with log4net. Some projects have found a way around this by pulling in the log4net file if it is there through `Assembly.Load`, otherwise using a null logger.