Fix .sass-cache location, switch to cwd + .sass-cache
Fix .sass-cache location, switch to cwd + .sass-cache

This change creates .sass-cache in the user's gulpfile directory. Previously it
was created in the gulp-ruby-sass node module directory.

mathiasbynens/gulp-ruby-sass,froucher/gulp-ruby-sass,sindresorhus/gulp-ruby-sass