Stop building against Ruby 2.1 and 2.2

Ruby 2.1 is has been officially obsolete for over two years now, and no
longer receives security updates:

https://www.ruby-lang.org/en/news/2017/04/01/support-of-ruby-2-1-has-ended/

Similarly Ruby 2.2 has been obsolete for over a year:

https://www.ruby-lang.org/en/news/2018/06/20/support-of-ruby-2-2-has-ended/

One impact of supporting older versions is that we can't keep our
development dependencies up-to-date. This adds some friction for
contributors.

Changelog format follows https://keepachangelog.com/en/1.0.0/

.travis.yml

@@ -6,8 +6,6 @@ matrix:
   include:
     - rvm: 2.5.3 # Pre-installed Ruby version
       script: bundle exec rake rubocop # ONLY lint once, first
-    - rvm: 2.1
-    - rvm: 2.2
     - rvm: 2.3.5
     - rvm: 2.4.6
     - rvm: 2.5.5

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Pundit
 
+## Unreleased
+
+### Removed
+
+- Dropped support for Ruby end-of-life versions: 2.1 and 2.2.
+
 ## 2.1.0 (2019-08-14)
 
 ### Fixed