Fixed RuboCop offense in Rakefile generated by bundle gem

```
Offenses:

Rakefile:18:1: C: [Correctable] Layout/EmptyLines: Extra blank line detected.

Diff:
@@ -11,4 +11,5 @@
   ext.lib_dir = "lib/test_gem"
 end

+
 task default: :compile

Author: sue445

bundler/lib/bundler/templates/newgem/Rakefile.tt

@@ -64,7 +64,6 @@ require "go_gem/rake_task"
 
 GoGem::RakeTask.new("<%= config[:underscored_name] %>")
 <% end -%>
-
 <% end -%>
 <% if default_task_names.size == 1 -%>
 task default: <%= default_task_names.first.inspect %>