Add CI check to ensure that gemspec is up to date

[movermeyer]

Problem

ruby-cldr.gemspec is auto-generated by bundle exec rake gemspec (via Jeweler).

However, developers will forget to run it as part of their PRs. Which means that we risk releasing a gem without all the required files, for example.

Potential solutions

a) add it to the checklist in the PR template
b) add a CI check that fails if bundle exec rake gemspec && rubocop -a ruby-cldr.gemspec produces any differences.