Fix Tilt warnings

[amoeba]

Haml::TempleEngine: Option :outvar is invalid
       /app/vendor/bundle/ruby/2.6.0/gems/tilt-1.4.1/lib/tilt/haml.rb:55: warning: Haml::Engine#precompiled_method_return_value at /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/forwardable.rb:158 forwarding to private method Haml::TempleEngine#precompiled_method_return_value
       Haml::TempleEngine: Option :outvar is invalid
       /app/vendor/bundle/ruby/2.6.0/gems/tilt-1.4.1/lib/tilt/haml.rb:55: warning: Haml::Engine#precompiled_method_return_value at /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/forwardable.rb:158 forwarding to private method Haml::TempleEngine#precompiled_method_return_value
       WARN: tilt autoloading 'sass' in a non thread-safe way; explicit require 'sass' suggested.
       Haml::TempleEngine: Option :outvar is invalid
       /app/vendor/bundle/ruby/2.6.0/gems/tilt-1.4.1/lib/tilt/haml.rb:55: warning: Haml::Engine#precompiled_method_return_value at /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/forwardable.rb:158 forwarding to private method Haml::TempleEngine#precompiled_method_return_value
       Haml::TempleEngine: Option :outvar is invalid
       /app/vendor/bundle/ruby/2.6.0/gems/tilt-1.4.1/lib/tilt/haml.rb:55: warning: Haml::Engine#precompiled_method_return_value at /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/forwardable.rb:158 forwarding to private method Haml::TempleEngine#precompiled_method_return_value