upgrade randomatic for yarn.lock vulnerability

[ElreyB]

@parkerziegler If I understood the vulnerability alert correctly it is a dependency in the yarn.lock file (randomatic) that needed to be upgrade. I ran yarn upgrade to fix this.

[ryan-roemer]

Side note: the PR has upgraded using a more modern yarn that also adds integrity sums like:

integrity sha512-cuAuTTIQ9RqcFRJ/Y8PvTh+paepNcaGxwQwjIDRWPXmzzyAeCO4KqS9ikMvq0MCbRk6GlYKwfzStrcP3/jSL8g==

This is definitely a good thing, but maybe we add a note wherever we talk about using / installing yarn to make sure to use yarn@1.11+ (it was 1.10 or something that introduced the hashes0.

[parkerziegler]

@ElreyB @ryan-roemer Our security issue appears to have gone away after our yarn upgrade introduced in #263. So I'm going to close this PR and, in working on some updated docs for Contributing, make sure to make note of our yarn requirements.