fix: update plugin semver to allow for rc versions

[Fdawgs]

Using ^4.0.0 causes the following error:

fastify-plugin: @fastify/elasticsearch - expected '^4.0.0' fastify version, '4.0.0-rc.3' is installed

You can see this occurring in the CI runs in #79

Checklist

• run npm run test and npm run benchmark
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm