CI: Drop 2.3, 2.4 from matrix

See support schedule of Ruby https://www.ruby-lang.org/en/downloads/branches/

Author: olleolleolle

.travis.yml

@@ -4,8 +4,6 @@ cache: bundler
 
 matrix:
  include:
- - rvm: 2.3
- - rvm: 2.4
  - rvm: 2.5
  - rvm: 2.6
  - rvm: 2.7