Point to python3 executable for rendering RST

#919 added support for python3, but
we still explicitly shell out to the `python2` executable.  Let's switch
to python3 now that it's supported.

This bumps the major version because it's a breaking change for anyone
running in an environment with only python2.

Author: Carl Brasic

lib/github-markup.rb

@@ -1,6 +1,6 @@
 module GitHub
   module Markup
-    VERSION = '3.0.5'
+    VERSION = '4.0.0'
     Version = VERSION
   end
 end

lib/github/markups.rb

@@ -49,7 +49,7 @@
 
 command(
   ::GitHub::Markups::MARKUP_RST,
-  "python2 -S #{Shellwords.escape(File.dirname(__FILE__))}/commands/rest2html",
+  "python3 -S #{Shellwords.escape(File.dirname(__FILE__))}/commands/rest2html",
   /re?st(\.txt)?/,
   ["reStructuredText"],
   "restructuredtext"