Update _bootswatch.scss

Sass::SyntaxError - wrong number of arguments (1 for 0) for `e'
  (gem) bootswatch-rails-0.4.0/vendor/assets/stylesheets/bootswatch/amelia/_bootswatch.scss:385:in `buttonBackgroundCustom'
  (gem) bootswatch-rails-0.4.0/vendor/assets/stylesheets/bootswatch/amelia/_bootswatch.scss:396:in `'