Use launchy for opening browsers

rakefile

@@ -601,15 +601,7 @@ task :showdocs, [:options] do |t, args|
     end
 
     Dir.chdir("#{path}/build/html") do
-        if RUBY_PLATFORM.include? 'darwin'  #  mac os
-            sh('open index.html')
-        elsif RUBY_PLATFORM.include? 'linux'  # make more ubuntu specific?
-            sh('sensible-browser index.html')  # ubuntu
-        else
-            raise "\nUndefined how to run browser on your machine.
-Please use 'rake builddocs' and then manually open
-'mitx/#{path}/build/html/index.html."
-        end
+        Launchy.open('index.html')
     end
 end