Update oga gem dependency to version 3.0

[pollinoco]

Oga gem release 2.16, 2.17 was removed. "For the reason why that version was removed, see issue #196 (closed). You should be able to update to v3.0 with no fear in most cases."

[thibaudgg]

Hi @pollinoco,

I'm no more actively maintaining that gem, but if you prepare a pull request with the Oga gem upgrade, I will happily merge it and release a new version.

[schugabe]

It would be great if you could release an update of this gem to fix the dependency issue.

[thibaudgg]

I just published the version 2.7.1 with Oga 3 support.

Thanks @andrewkendall for the fix.

Closed by #187