Remove 2.4 (EOL)

See https://www.ruby-lang.org/en/news/2020/04/05/support-of-ruby-2-4-has-ended/

Author: tianon

.travis.yml

@@ -18,12 +18,6 @@ env:
   - VERSION=2.5 VARIANT=stretch/slim
   - VERSION=2.5 VARIANT=alpine3.11
   - VERSION=2.5 VARIANT=alpine3.10
-  - VERSION=2.4 VARIANT=buster
-  - VERSION=2.4 VARIANT=buster/slim
-  - VERSION=2.4 VARIANT=stretch
-  - VERSION=2.4 VARIANT=stretch/slim
-  - VERSION=2.4 VARIANT=alpine3.11
-  - VERSION=2.4 VARIANT=alpine3.10
 
 install:
   - git clone --depth 1 https://github.com/docker-library/official-images.git ~/official-images