StackFrame#from_exception should not fail with native java exceptions

Currently on JRuby versions 1.6.8, 1.7.0, 1.7.1, 1.7.2, You can run into error:

```ruby
undefined method `__better_errors_bindings_stack' for java.lang.Exception: oops:Java::JavaLang::Exception
     # ./lib/better_errors/stack_frame.rb:5:in `from_exception'
```

This is simple fix to not fail with *undefined method*. I've added missing method to all native java exceptions - for now it returns empty array, just to fallback to *#backtrace* in *StackFrame#from_exception*.

I also updated *.gemspec* work with jruby 1.6.8.

I'm not sure how to write specs for that kind of 'platform specific' features. Maybe it would be good idea to put them into separate file? Any suggestions are welcome.