Rake notes compatibility in Rails 5.1.0 (#1661)

* As of Rails 5.1.0 gems can register a directories with the
Rails `SourceAnnotationExtractor` which means when we 
run `rake notes` it will now also list out any todos in the
`/spec` folder.

This was added in this PR:
rails/rails#25692

Resulting in:

```bash

$ rake notes
app/spec/models/article_spec.rb:
  * [2] [TODO] add more specs
  * [3] [FIXME] Spec A is broken
  * [4] [OPTIMIZE] improve this test

```

* In this commit:
* just fixed the railtie

As of Rails 5.1.0 gems can register a directories to work with `rake
notes`

In this case we can tell Rails it to now look in the `/spec` folder when
searching for annotations like `# TODO:` `# FIXME` etc.

This was added in this PR:
rails/rails#25692

Resulting in:

```bash

$ rake notes
app/spec/models/article_spec.rb:
  * [2] [TODO] add more specs
  * [3] [FIXME] Spec A is broken
  * [4] [OPTIMIZE] improve this test

```

Author: johnmeehan

lib/rspec-rails.rb

@@ -8,6 +8,10 @@ module RSpec
   module Rails
     # Railtie to hook into Rails.
     class Railtie < ::Rails::Railtie
+      # As of Rails 5.1.0 you can register directories to work with `rake notes`
+      if ::Rails::VERSION::STRING >= '5.1'
+        SourceAnnotationExtractor::Annotation.register_directories("spec")
+      end
       # Rails-3.0.1 requires config.app_generators instead of 3.0.0's config.generators
       generators = config.respond_to?(:app_generators) ? config.app_generators : config.generators
       generators.integration_tool :rspec