Update gumbo to d6b952b4 to pick up another entity parsing fix

[rgrove]

Would you mind updating gumbo to commit d6b952b4bcfa15c8f9bc28f56457b969e7f96ffe to pick up another entity parsing fix?

I'd submit a PR, but I've gotten the impression that PRs containing subproject commit bumps cause you more work than they save you. Happy to submit one if you'd prefer.

[rubys]

Thanks!

Background on the problem I've had: GitHub provides a button I can use to merge a pull request. Unfortunately, when I then pull the updated master down to my machine, it doesn't update the gumbo submodule version. What's worse is that if I later push, that causes the submodule version to be updated. Not sure why it happens.

[rgrove]

Thanks!