Update changelog task

- require the gem in Gemfile
- update for main branch

Author: tvdeyen

Gemfile

@@ -14,3 +14,5 @@ group :test do
     gem "codeclimate-test-reporter", "~> 1.0", require: false
   end
 end
+
+gem "github_fast_changelog"

Rakefile

@@ -48,7 +48,7 @@ BASH
       original_file = './CHANGELOG.md'
       new_file = original_file + '.new'
       backup = original_file + '.old'
-      changes = `git rev-list #{ENV['PREVIOUS_VERSION']}...main | bundle exec github_fast_changelog AlchemyCMS/alchemy-devise`
+      changes = `git rev-list v#{ENV['PREVIOUS_VERSION']}...main | bundle exec github_fast_changelog AlchemyCMS/alchemy-devise`
       File.open(new_file, 'w') do |fo|
         fo.puts changes
         File.foreach(original_file) do |li|