Tweak lock file to pull in only one React version
Yarn deps can be a bit fiddly when targeting specific versions. This tweaks the
lock file to only pull in a single React version.
For others who may need to do something like this again, I recommend deleting
specific package entries from `yarn.lock` and re-running `yarn install`. That
should make it pick the right thing while also leaving other packages as they
were.
