ci: Ensure Appraisal fetches latest version of dependencies

[marcotc]

appraisal install runs bundle check first, then bundle install if bundle check returned false:

bundle exec appraisal install
>> bundle check --gemfile='gemfiles/rails5_mysql2.gemfile' || bundle install --gemfile='gemfiles/rails5_mysql2.gemfile'

bundle check returns true if the gems currently installed in the system can fulfill the Gemfile.lock (or simply the Gemfile, if a lock file is absent).

When bundle check runs the for first time in our CI, there is no Gemfile.lock available, so it tries to satisfy the Gemfile dependencies, which are lenient (e.g. gem 'excon'), and bundle check will consider that dependency fulfilled as long as a version that satisfies the version constraint is installed.

In contrast, bundle install will always try to fetch the latest version from the remote, and will skip downloading it if it is already present.

Unfortunately, there's no API to invoke appraisal install without bundle check.

This PR cleans up any existing appraisal Gemfiles, with appraisal clean, which ensures bundle check returns false (without a Gemfile, bundle check fails), and ensuring bundle install runs for all Appraisal combinations.