[1-12-1] Update ruobocop-rails to 2.9.1 (#292)

Author: f-moya

Gemfile.lock

@@ -60,10 +60,10 @@ GEM
       rubocop (>= 0.74)
     rubocop-performance (1.7.0)
       rubocop (>= 0.82.0)
-    rubocop-rails (2.7.1)
+    rubocop-rails (2.9.1)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
-      rubocop (>= 0.87.0)
+      rubocop (>= 0.90.0, < 2.0)
     rubocop-rake (0.5.1)
       rubocop
     rubocop-rspec (1.41.0)