Update ruby version support to include 2.6

.travis.yml

@@ -13,6 +13,7 @@ rvm:
   - 2.3.8
   - 2.4.5
   - 2.5.3
+  - 2.6.0
 
 branches:
   only:

vagrant.gemspec

@@ -12,7 +12,7 @@ Gem::Specification.new do |s|
   s.summary       = "Build and distribute virtualized development environments."
   s.description   = "Vagrant is a tool for building and distributing virtualized development environments."
 
-  s.required_ruby_version     = "~> 2.2", "< 2.6"
+  s.required_ruby_version     = "~> 2.2", "< 2.7"
   s.required_rubygems_version = ">= 1.3.6"
   s.rubyforge_project         = "vagrant"