Add ruby 2.6.3 to travis
Add ruby 2.6.3 to travis

gavv/rdoc-generator-singlepage,rbdoc/rdoc-generator-solarfish,gavv/rdoc-generator-singlepage,rbdoc/rdoc-generator-singlepage,rbdoc/rdoc-generator-solarfish,rbdoc/rdoc-generator-singlepage