use path.join for destFile

The destFile ended up being something like `tmp/sass_compiler-tmp_cache_dir-ILqw5MKk.tmpapp.css` instead of `tmp/sass_compiler-tmp_cache_dir-ILqw5MKk.tmp/app.css`. Using `path.join` should fix this, or am I missing something? 

I make a quick demo repo to outline the issue https://github.com/moudy/broccoli-sass-output-file-test/blob/master/Brocfile.js