Drop ruby2.4 support

Mostly because #delete_suffix and #delete_prefix
methods are not available in Ruby 2.4.x and were
added in Ruby 2.5.x.

Signed-off-by: Utkarsh Gupta <utkarsh@debian.org>

.travis.yml

@@ -3,7 +3,6 @@ language: ruby
 cache: bundler
 before_install: gem install bundler -v 2.1.4
 rvm:
-  - 2.4.10
   - 2.5.8
   - 2.6.6
   - 2.7.1