after_commit should not be called on a failed Model.create! in Rails 3

I fixed a bug where after_commit as being called on a Rails 3 project from a failed Model.create! call.

You can see I just added `ActiveRecord::RecordInvalid` to the list of exceptions to catch.

I also updated the Readme.md file with instructions on how to test since I've never heard of wwtd and couldn't figure out why running `bundle exec rspec` failed. This should save the next contributor some time.