third-party: update libyaml submodule

Need to update tests as with fixup in upstrem
commit baf636a74b4b6d055d93e2d01366d6097eb82d90
Author: Tina Müller <cpan2@tinita.de>
Date:   Thu Jun 14 19:27:04 2018 +0200

The closing single quote needs to be indented...
if it's on its own line.

Closes #3275.

test/app/digest.result

@@ -333,7 +333,7 @@ b
 
   YWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFh
 
-'
+  '
 ...
 digest.base64_decode(b) == s
 ---

test/app/socket.result

@@ -2115,7 +2115,7 @@ c:receive("*a", "Message: ")
 ---
 - 'Message: Pong
 
-'
+  '
 ...
 c:receive("*a", "Message: ")
 ---