added dependency to sass-rails and changed port for previewing

Hi Gunnar,

I had to change two configurations for getting mapstruct.org running on local machine. First I had to change the port in site.yml to get JS and CSS imports done and I had to add an explicit dependency to sass-rails to the Rakefile. 

Can you please take a look at this PR. I guess I did something wrong, when setting up mapstruct.org locally and I would like to document this pitfall.