Opt out of SuggestExtensions message

This PR suppresses the following `SuggestExtensions` message.

```console
% bundle exec rake
(snip)

Tip: Based on detected gems, the following RuboCop extension libraries
might be helpful:
  * rubocop-rake (http://github.com/rubocop-hq/rubocop-rake)

You can opt out of this message by adding the following to your
config (see
https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions
for more options):
  AllCops:
    SuggestExtensions: false
```

.rubocop.yml

@@ -13,6 +13,7 @@ AllCops:
     - 'spec/fixtures/**/*'
     - 'tmp/**/*'
   TargetRubyVersion: 2.4
+  SuggestExtensions: false
 
 Naming/PredicateName:
   # Method define macros for dynamically generated method.