Rails Instrumentation should honor the ignored_environments config variable

Having specified `config.ignored_environments = ['development', 'test']` I was surprised to see errplane attempting to send instrumentation of test runs. After looking at [the code](https://github.com/errplane/errplane-ruby/blob/master/lib/errplane/rails/instrumentation.rb) it doesn't look like the Errplane::Rails::Instrumentation checks if the current `Rails.env` is ignored. It really should.
