Add missing rubocop dependencies

Rails and performance cops have been extracted to separate gems.

Note in the past we had to add these lines in order to activate Rails
cops:

```
Rails:
  Enabled: true
```

But we didn't do it, and so Rails cops were ignored.

With the new version, it's enough to require `rubocop-rails`.

.rubocop_basic.yml

@@ -1,4 +1,7 @@
-require: rubocop-rspec
+require:
+  - rubocop-performance
+  - rubocop-rails
+  - rubocop-rspec
 
 AllCops:
   DisplayCopNames: true

Gemfile

@@ -97,6 +97,8 @@ group :development do
   gem "erb_lint", require: false
   gem "mdl", "~> 0.5.0", require: false
   gem "rubocop", "~> 0.75.0", require: false
+  gem "rubocop-performance", "~> 1.4.1", require: false
+  gem "rubocop-rails", "~> 2.3.2", require: false
   gem "rubocop-rspec", "~> 1.35.0", require: false
   gem "rvm1-capistrano3", "~> 1.4.0", require: false
   gem "scss_lint", "~> 0.55.0", require: false

Gemfile.lock

@@ -466,6 +466,11 @@ GEM
       rainbow (>= 2.2.2, < 4.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 1.7)
+    rubocop-performance (1.4.1)
+      rubocop (>= 0.71.0)
+    rubocop-rails (2.3.2)
+      rack (>= 1.1)
+      rubocop (>= 0.72.0)
     rubocop-rspec (1.35.0)
       rubocop (>= 0.60.0)
     ruby-progressbar (1.10.1)
@@ -647,6 +652,8 @@ DEPENDENCIES
   rollbar (~> 2.18.0)
   rspec-rails (~> 3.8)
   rubocop (~> 0.75.0)
+  rubocop-performance (~> 1.4.1)
+  rubocop-rails (~> 2.3.2)
   rubocop-rspec (~> 1.35.0)
   rvm1-capistrano3 (~> 1.4.0)
   sass-rails (~> 5.0, >= 5.0.4)