Fixed mixins skipping the next declaration

Since we are removing one of the declarations, we need to decrement `i` to check the current index again (since the for loop will just re-increment it).

This ends up being an issue if you have two mixins right after another. For example, I had:

    grid-columns: 16
    grid-position: alpha

when using [rework-grid](http://github.com/rschmukler/rework-grid).