[rubygems/rubygems] Fix Rakefile to make Style/SymbolArray happy

In case of multiple Rake tasks, the default tasks would
look something like this:
`task default: [:spec, :rubocop]`

Instead, they should use %i and look something like this:
`task default: %i[spec rubocop]`

Signed-off-by: Utkarsh Gupta <utkarsh@debian.org>

https://github.com/rubygems/rubygems/commit/487ecd59ce

lib/bundler/templates/newgem/Rakefile.tt

@@ -36,4 +36,8 @@ Rake::ExtensionTask.new("<%= config[:underscored_name] %>") do |ext|
 end
 
 <% end -%>
-task default: <%= default_task_names.size == 1 ? default_task_names.first.inspect : default_task_names.inspect %>
+<% if default_task_names.size == 1 -%>
+task default: <%= default_task_names.first.inspect %>
+<% else -%>
+task default: %i[<%= default_task_names.join(" ") %>]
+<% end -%>