Add Ruby 2.4.5 and 2.5.3 to build matrix (#775)
Add Ruby 2.4.5 and 2.5.3 to build matrix (#775)

* Add Ruby 2.4.5, 2.5.3, and 2.6.1 to build matrix.

* Allow failures for Ruby 2.5.3 and Rails 3.0

An error related `yaml_as` occurs in this combination, because Ruby 2.5 removed `yaml_as` method.

mileszs/wicked_pdf,bonekost/wicked_pdf