chore: replace ri_cal with icalendar

based upon rubyredrick/ri_cal#22 (comment) i assume phoet intended to replace the quite outdated ri_cal with icalendar a while ago.
so doing it now :)

Gemfile

@@ -22,7 +22,7 @@ gem 'public_suffix'
 gem 'puma'
 gem 'rack-cache'
 gem 'redcarpet'
-gem 'ri_cal', github: 'phoet/ri_cal', branch: 'master'
+gem 'icalendar'
 gem 'simple_form'
 gem 'slim-rails'
 gem 'sprockets', '~> 3.7'

Gemfile.lock

@@ -1,12 +1,3 @@
-GIT
-  remote: https://github.com/phoet/ri_cal.git
-  revision: 0c607408f931ebbbed1c5e4dd6e4eb22572f268f
-  branch: master
-  specs:
-    ri_cal (0.8.8)
-      activesupport (> 2.2)
-      tzinfo (> 2.0)
-
 GEM
   remote: http://rubygems.org/
   specs:
@@ -135,6 +126,9 @@ GEM
     hashie (5.0.0)
     i18n (1.14.4)
       concurrent-ruby (~> 1.0)
+    icalendar (2.10.2)
+      ice_cube (~> 0.16)
+    ice_cube (0.17.0)
     image_info (1.2.2)
       addressable (~> 2.0)
       image_size (~> 2.0)
@@ -459,6 +453,7 @@ DEPENDENCIES
   faker
   font-awesome-rails
   geocoder
+  icalendar
   kaminari
   kgio
   letter_opener
@@ -480,7 +475,6 @@ DEPENDENCIES
   rails-controller-testing
   rails_12factor
   redcarpet
-  ri_cal!
   rspec-collection_matchers
   rspec-rails
   rubocop

app/helpers/ical_helper.rb

@@ -2,18 +2,18 @@
 
 module IcalHelper
   def icalendar(*events)
-    RiCal.Calendar do |cal|
-      events.each do |event|
-        cal.event do |item|
-          item.summary     = event.name
-          item.description = "#{event.description} #{event_url(event)}"
-          item.dtstart     = event.date
-          item.dtend       = event.end_date
-          item.url         = event_url(event)
-          item.location    = event.location.name if event.location
-        end
+    cal = Icalendar::Calendar.new
+    events.map do |event|
+      cal.event do |item|
+        item.summary     = event.name
+        item.description = "#{event.description} #{event_url(event)}"
+        item.dtstart     = event.date
+        item.dtend       = event.end_date
+        item.url         = event_url(event)
+        item.location    = event.location.name if event.location
       end
-    end.to_s
+    end
+    cal.to_ical
   end
 
   def calendar_link