Add appraisal for Rails 6 beta

Appraisals

@@ -21,3 +21,7 @@ end
 appraise "rails_5_2" do
   gem "rails", "5.2"
 end
+
+appraise "rails_6.beta1" do
+  gem "rails", "6.0.0.beta1"
+end

gemfiles/rails_6.beta1.gemfile

@@ -0,0 +1,10 @@
+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "rubysl", "~> 2.0", :platforms => :rbx
+gem "minitest", :platforms => :rbx
+gem "html2haml"
+gem "rails", "6.0.0.beta1"
+
+gemspec :path => "../"