Fix simplex/_bootswatch.scss for box-shadow error

I had this error when using the simplex template with bootswatch-rails 0.3.1, rails 3.2.11 and ruby 1.9.2:

    Mixin box-shadow takes 1 argument but 2 were passed. 

The reason was that "to pass multiple values to the Bootstrap mixins, you must escape the values or else the Sass parser will choke on the commas" (https://github.com/thomas-mcdonald/bootstrap-sass#passing-multiple-values-to-mixins).