Update index.html download button to 2.0.0.

Author: matz

index.html

@@ -11,7 +11,7 @@
     </p>
 
     <div class="mruby-is-get">
-      <a href="https://github.com/mruby/mruby/archive/1.4.1.zip" class="btn btn-lg btn-default" role="button">Download 1.4.1 Source</a>
+      <a href="https://github.com/mruby/mruby/archive/2.0.0.zip" class="btn btn-lg btn-default" role="button">Download 2.0.0 Source</a>
       &nbsp; or &nbsp;
       <a href="https://github.com/mruby/mruby" class="btn btn-lg btn-default" role="button">Find on GitHub</a>
     </div>