:bug: Fix usemin directive in index.html
:bug: Fix usemin directive in index.html

`main.css` will never be in the app folder, only the main.scss.
But `main.css` is in `dist` when build with gulp, and in `.tmp`
when build with grunt.

despairblue/scegratoo3,despairblue/scegratoo3,despairblue/scegratoo3