Add some more buffer for timeout check

This spec is unstable on our CI environment:
https://gist.github.com/ko1/efa58acf4cd1d7b5bf1f63881537afe8

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67373 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/library/timeout/timeout_spec.rb

@@ -26,7 +26,7 @@ describe "Timeout.timeout" do
       end
     end.should raise_error(StandardError)
 
-    (Time.now - before_time).should be_close(1.0, 0.5)
+    (Time.now - before_time).should be_close(1.0, 0.8)
   end
 
   it "returns back the last value in the block" do