Add the google maps javascript tag to the layout header

# app/views/layouts/my_layout.html.haml
!!!
%html{:lang => 'en'}
  %head
    - # ...
    = google_maps_api_script_tag

# app/helpers/google_maps_helper.rb
module GoogleMapsHelper
  def google_maps_api_script_tag
    javascript_include_tag google_maps_api_source
  end

  def google_maps_api_source
    "https://maps.googleapis.com/maps/api/js?key=#{google_maps_api_key}"
  end

  def google_maps_api_key
    Rails.application.secrets.google_maps_api_key
  end
end

# config/secrets.yml
development:
  google_maps_api_key: '...'
  # ...
production:
  google_maps_api_key: '...'
  # ...