Add Ruby 2.1 and 2.2 to CI matrix again

Let's avoid the issues with bundler > 2 for now.

.travis.yml

@@ -1,11 +1,13 @@
 language: ruby
 before_install:
-  - gem install bundler
+  - gem install bundler -v 1.17.3
 
 matrix:
   include:
     - rvm: 2.5.1
       script: bundle exec rake rubocop # ONLY lint once, first
+    - rvm: 2.1
+    - rvm: 2.2
     - rvm: 2.3.5
     - rvm: 2.4.2
     - rvm: 2.5.1

CHANGELOG.md

@@ -8,7 +8,6 @@ None
 
 ### Other changes
 
-- Drop official support for Ruby 2.1 and 2.2. (#573)
 - Improve exception handling for `#policy_scope` and `#policy_scope!`. (#550)
 - Add `:policy` metadata to RSpec template. (#566)