Bump snakeyaml version to 1.11

[briprowe]

Upgrading snakeyaml to v1.11 fixes an issue with parsing timestamps with greater than millisecond precision. For example: "2001-11-23 15:02:31.123456 -04:00" would parse incorrectly as "2001-11-23T19:04:34.456-00:00"

[startling]

Hey, it seems like this library's unmaintained. We've forked it at circleci/clj-yaml, fixed some bugs, and bumped the snakeyaml version up to 1.13. Feel free to use it. I've added your test, hope you don't mind? :)