This fixes segmentation fault in the tests as extending the examples in a before block with override of `#instance_eval` seems to not work. Also, the context wrap is now opt-in with a helper method, instead of opt-out and relying on `#described_class` (which has slightly different behavior in RSpec 3). Tested on Ruby 2.1.5 and RSpec 2.99.2.
This increases the discoverability of what license this gem uses. See rubygems/rubygems.org#363 or [this blog post](http://www.benjaminfleischer.com/2013/07/12/make-the-world-a-better-place-put-a-license-in-your-gemspec/) for more info.