Fix ical formatter, specs, update icalendar gem, add gh-action

[mralexandernickel]

• Update icalendar gem to latest version
• Adapt changes form icalendar-gem in formatter
• Adapt changes from icalendar-gem in specs
• Add gh-action to execute rspec on push and pull_request

[mralexandernickel]

Alright, I guess the real issue here and in #163 is in fact a completely different. For me the export to ical throwed the same error as reported in #163, but at first I got the Error that the "icalendar gem" is needed for this Formatter to work:

https://github.com/samg/timetrap/blob/master/lib/timetrap/formatters/ical.rb#L4-L8

So I installed icalendar on my system, without passing a specific version, and that ended in the error reported by #163

I still think this PR is helpful, because the formatter is working with latest icalendar-version after adding some changes. And also a "required check" in PRs via the github-action could make maintaining this project easier. But right now it seems to me the real issue is: the dependencies like icalendar aren't installed, when one installs timetrap simply via gem install timetrap on the system.

[categulario]

thanks a lot for the github actions!