Read stderr from cpu count command to avoid hang

Leaving stderr unread causes hangs.  I don't remember why, though, the
documentation somewhere probably says.

See #268

History.rdoc

@@ -13,6 +13,8 @@ Bug fixes:
 * Fixed typo, removed extra "h".  Pull request #267 by Hsing-Hui Hsu.
 * Rake no longer reports an error when cleaning already-deleted files.  Pull
   request #266 by Randy Coulman.
+* Consume stderr while determining CPU count to avoid hang.  Issue #268 by
+  Albert Sun.
 
 === 10.2.2 / 2014-03-27
 

lib/rake/cpu_counter.rb

@@ -79,7 +79,9 @@ def count_via_sysctl
     def run(command, *args)
       cmd = resolve_command(command)
       if cmd
-        Open3.popen3 cmd, *args do |_, out|
+        Open3.popen3 cmd, *args do |inn, out, err,|
+          inn.close
+          err.read
           out.read.to_i
         end
       else