Remove encoding dance to avoid JRuby bug

Hi,
There is a [bug in JRuby](https://github.com/jruby/jruby/issues/375) that prevents changing string encoding with the `:invalid => :replace` option.  The JRuby developers have indicated a fix is not easy (and presumably unlikely to come any time soon).

This patch removes these calls from `exception_notification`, but obviously they were added for a reason.  Unfortunately, I have been [unable to reproduce](https://github.com/smartinez87/exception_notification/issues/113#issuecomment-12907925) the errors that prompted this code's inclusion in the first place. (#52, #86).

As it is, with my pull request in place, all unit tests pass in Ruby 1.8.7, Ruby 1.9.3, and JRuby 1.7.2.  Without my patch, the tests fail on JRuby.

Do you have any way of reproducing #52 or #86?  It'd be great if there was a unit test in place, then I could work around the JRuby issue without breaking MRI edge cases.  I'd really like to get this working JRuby.