…endencies

…g .rspec file triggers loading autotest/rspec2.

…is not an override of the original recurring event.

…rrence_id set) which act as exceptions/overrides for a recurring event.

…rences for a complete calendar. This allows implementation of specific instances with modified properties overriding an occurrence of a recurring event.

…moved into range by specific instance,

…cy by (correctly) scoping to individual UID's.

- delete .ruby-version/gemset
- delete travis.yml
- delete empty .rspec
- improve gemspec

See: https://github.com/thoughtbot/appraisal

drop support for tzinfo < 2
this also drops support for active_support < 6.1

See: planningcenter@d9d8584

See: planningcenter@66f9e7c

See: planningcenter@58f5f34

See: planningcenter@d564957

fix `tzinfo 2.0` regressions