Support new flycheck

I updated flycheck and helm-flycheck was broken.

Reason list
･ deleted function flycheck-sort-errors by [Drop redundant flycheck-sort-errors](https://github.com/flycheck/flycheck/commit/603ac4ef965868cc5c5bc75bdcc564605ee3cf43)
･ changed return value type by [Restore mouse support in error list](https://github.com/flycheck/flycheck/commit/5441d3d2d0703511310b389b83e347d97f4f20d5)

I write suppport patch.