language: ruby
rvm:
  - 2.2.9
  - 2.3.6
  - 2.4.3
  - 2.5.0
before_install: gem install bundler -v 1.16.1

language: ruby
rvm:
  - 2.4.5
  - 2.5.4
  - 2.6.1
before_install: gem install bundler -v 1.16.1

Update supported Ruby versions to Ruby >= 2.4
Update supported Ruby versions to Ruby >= 2.4

This removes support for Ruby < 2.4 and adds Travis CI configurations
for the most recent minor versions >= 2.4 (2.4.5, 2.5.4, 2.6.1).

Support for Ruby 2.3 has already ended and 2.4 will reach its end of
life at the end of March 2019.
