Fix version label behavior for PRs

[t1m0thyj]

• Fixes version labels not detected on a PR that has more than 30 events in its history due to GitHub API pagination (example). The workaround in this PR is to set per_page to the max allowed value of 100, and hope that our PRs never exceed that number of events. Considered other options like using the Octokit paginate API to retrieve an unlimited number of events, or issuing GraphQL queries to filter the event list, but neither method supports caching so rate limiting would be a problem.
• Fixes the version number in package.json being ignored when the GitHub plugin bumps the version. Since the NPM plugin stores the package.json version property in context.version.new, we should prefer it over context.version.old when it is defined. This should bump versions correctly even if past Git tags point to detached commits (example).

[zFernand0]

LGTM! 😋