Tell older bundlers which default branch to use

Author: mattwynne

Gemfile

@@ -7,13 +7,13 @@ gemspec
 if ENV['CUCUMBER_RUBY_CORE']
   gem 'cucumber-core', path: ENV['CUCUMBER_RUBY_CORE']
 elsif !ENV['CUCUMBER_USE_RELEASED_GEMS']
-  gem 'cucumber-core', github: 'cucumber/cucumber-ruby-core'
+  gem 'cucumber-core', github: 'cucumber/cucumber-ruby-core', branch: 'main'
 end
 
 if ENV['CUCUMBER_RUBY_WIRE']
   gem 'cucumber-wire', path: ENV['CUCUMBER_RUBY_WIRE']
 elsif !ENV['CUCUMBER_USE_RELEASED_GEMS']
-  gem 'cucumber-wire', github: 'cucumber/cucumber-ruby-wire'
+  gem 'cucumber-wire', github: 'cucumber/cucumber-ruby-wire', branch: 'main'
 end
 
 gem 'cucumber-expressions', path: ENV['CUCUMBER_EXPRESSIONS_RUBY'] if ENV['CUCUMBER_EXPRESSIONS_RUBY']