Log Message:
(geography/R-stars) Updated 0.6.7 to 0.6.8

# version 0.6-8

* address difftime issue new in R-devel rev 87670

* `c.stars()` is more strict when combining time sequences; #703

* fix plotting when breaks contain duplicates; #728

* fix `st_as_stars.im()`; #727 and #648, thanks to Barry Rowlingson