Add support for java exceptions in jruby.

## Problem

In jruby, if a java exception is raised, then none of the exception information will be included, since Airbrake.build_notice_for checks for `if exception.is_a?(Exception)` which returns `false` for a java.lang.Exception.

jruby has java extensions to make java.lang.Exception behave as a ruby Exception, but the backtrace doesn't use the same format, so `Airbrake::Notice.new(Airbrake.configuration.merge(exception: java_exception))` will pick up the error class and message, but all the fields in the backtrace will be blank.

## Solution

This is a proof of concept for creating an Airbrake::Notice from a java exception.

Is there any interest in having jruby specific code in the airbrake gem?  Or are all jruby users expected to build an Airbrake::Notice in this way like this in their application code?

The code could probably be made cleaner, by allowing a Airbrake::Backtrace to be provided as an option to Airbrake::Notice.new.