You can try this:
library(dplyr)
ranges %>%
arrange(start) %>%
group_by(g = cumsum(cummax(lag(stop, default = first(stop))) < start)) %>%
summarise(start = first(start), stop = max(stop))
# A tibble: 2 × 3
# g start stop
# <int> <dbl> <dbl>
#1 0 65.72000 87.75625
#2 1 89.61625 104.94062