Update github_changelog_generator requirement

Updates the requirements on [github_changelog_generator](https://github.com/github-changelog-generator/Github-Changelog-Generator) to permit the latest version.
- [Release notes](https://github.com/github-changelog-generator/Github-Changelog-Generator/releases)
- [Changelog](https://github.com/github-changelog-generator/github-changelog-generator/blob/master/CHANGELOG.md)
- [Commits](github-changelog-generator/github-changelog-generator@v1.15.0.pre.rc...v1.15.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

Gemfile

@@ -1,4 +1,4 @@
 source 'https://rubygems.org' do
   gem 'bundler', '2.0.2'
-  gem 'github_changelog_generator', '1.15.0.pre.rc'
+  gem 'github_changelog_generator', '1.15.0'
 end