Bump rubocop from 1.9.0 to 1.9.1

Bumps [rubocop](https://github.com/rubocop-hq/rubocop) from 1.9.0 to 1.9.1.
- [Release notes](https://github.com/rubocop-hq/rubocop/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop-hq/rubocop/compare/v1.9.0...v1.9.1)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -215,7 +215,7 @@ GEM
     mnemosyne-ruby (1.11.0)
       activesupport (>= 4)
       bunny
-    msgpack (1.4.1)
+    msgpack (1.4.2)
     multi_json (1.15.0)
     multi_xml (0.6.0)
     multipart-post (2.1.1)
@@ -352,7 +352,7 @@ GEM
       rspec-mocks (~> 3.10)
       rspec-support (~> 3.10)
     rspec-support (3.10.2)
-    rubocop (1.9.0)
+    rubocop (1.9.1)
       parallel (~> 1.10)
       parser (>= 3.0.0.0)
       rainbow (>= 2.2.2, < 4.0)