Remove Ruby 2.3 (EOL)

See https://www.ruby-lang.org/en/news/2019/03/31/support-of-ruby-2-3-has-ended/ (official EOL announcement saying there will be no more fixes in 2.3).

Author: tianon

.travis.yml

@@ -16,12 +16,6 @@ env:
   - VERSION=2.4 VARIANT=jessie/slim
   - VERSION=2.4 VARIANT=alpine3.9
   - VERSION=2.4 VARIANT=alpine3.8
-  - VERSION=2.3 VARIANT=stretch
-  - VERSION=2.3 VARIANT=stretch/slim
-  - VERSION=2.3 VARIANT=jessie
-  - VERSION=2.3 VARIANT=jessie/slim
-  - VERSION=2.3 VARIANT=alpine3.8
-  - VERSION=2.3 VARIANT=alpine3.7
 
 install:
   - git clone --depth 1 https://github.com/docker-library/official-images.git ~/official-images