Update template.rb to work with haml 4

In version 4 and higher of haml, the Haml engine doesn't have a method `precompiled_with_ambles`. This method is implemented in the Haml::Compiler class, and every engine has `compiler` attribute of that class. This change is needed to make the tests pass with haml 4. It reflects the definition of the `def_method` method in `Haml::Engine`.