Add sass-cache directories to gitignore
Remove Ruby specific syntax from filter attribute

Using #{ruby_variable_interpolation} means that this file can't be
compiled using node-sass.

Change the value to use Sass arithmetic, string concatenation and
the unquote method to avoid this Ruby specific syntax.

http://sass-lang.com/documentation/Sass/Script/Functions.html#unquote-instance_method
Merge pull request #98 from alphagov/remove-ruby-syntax-filter-opacity

Remove Ruby specific syntax from filter attribute