Make preview task work on things other than macos

.rubocop.yml

@@ -5,3 +5,6 @@ inherit_gem:
 inherit_mode:
   merge:
     - Exclude
+
+Style/EmptyCaseCondition:
+  Enabled: false

Rakefile

@@ -1,3 +1,4 @@
+require "mkmf"
 require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 
@@ -13,7 +14,15 @@ task :generate_example do
   html = GovukMarkdown.render(markdown)
 
   File.write("example/example.html", ERB.new(File.read("example/example_layout.html.erb")).result(binding))
-  sh "open example/example.html"
+
+  case
+  when find_executable("xdg-open")     # linux
+    sh("xdg-open example/example.html")
+  when find_executable("open")         # mac
+    sh("open example/example.html")
+  else
+    puts "View the example in example/example.html"
+  end
 end
 
 desc "Print the current version of the gem"