Avoid removing files not ended in .rb

[pacop]

When the option compiled_ruby_files is personalized to use different extensions, for instance, .rake files. They are removed each time warble is called.

Problem was in lib/warbler/task.rb where every file not ended in .rb were being removed.

[pacop]

I was not able to pass specs, neither in travis nor my pc. Force-pushed to try again, but it was unsuccessful