sudo: false
language: ruby
rvm:
 - 1.9
 - 2.1
 - 2.2
 - 2.3
 - 2.4
 - 2.5

sudo: false
language: ruby
rvm:
 - 2.1
 - 2.2
 - 2.3
 - 2.4
 - 2.5
 - 2.6

Drop Ruby 1.9, Add Ruby 2.6
Travis: Drop Ruby 1.9, Add Ruby 2.6

Ruby 1.9 has been EOL for over 3 years, so I think it's fine to remove
it from the travis matrix.

This also adds Ruby 2.6 to ensure it continues to work in the future.
