Support nested exceptions for Ruby 2.1. Fixes #118.

I've added support for nested exceptions, which where added in Ruby 2.1.

The code checks if the reported exception responds to the `#cause` method and then loops over the cause exceptions generating an array of traces.

If the number of traces is 1, we send the payload using `trace` and we use `trace_chain` if the number of traces is more than 1.