fix(checkout_file.rb): explicitly use master branch

Signed-off-by: Norwid Behrnd <[email protected]>

levels/checkout_file.rb

@@ -14,6 +14,7 @@
   File.open("config.rb", "a") do |file|
     file.puts("These are changed you don't want to keep!")
   end
+  system "git branch -m master"
 end
 
 solution do