Add a missing require for octokit/repository

The Repository constant is not autoloaded nor eagerly loaded in
Octokit v4.23.

Author: elia

lib/solidus_dev_support/rake_tasks.rb

@@ -78,6 +78,7 @@ def install_changelog_task
 
       GitHubChangelogGenerator::RakeTask.new(:changelog) do |config|
         require 'octokit'
+        require 'octokit/repository'
         repo = Octokit::Repository.from_url(gemspec.metadata['source_code_uri'] || gemspec.homepage)
 
         config.user = repo.owner