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.

proiel/proiel,proiel/proiel