update gem dependencies to run on Ruby 2.5

The gemspec still says we're compatible with it, so lets try to make
this work.

Author: bastelfreak

Gemfile

@@ -5,7 +5,7 @@ source ENV['GEM_SOURCE'] || 'https://rubygems.org'
 gemspec
 
 group(:release, optional: true) do
-  gem 'faraday-retry', '~> 2.1', require: false
+  gem 'faraday-retry', '~> 2.1', require: false if RUBY_VERSION >= '2.6'
   gem 'github_changelog_generator', '~> 1.16.4', require: false
 end