Fix the calculation for development time so it doesn't come up with things like ...

Fix the calculation for development time so it doesn't come up with things like "2 months" between two dev releases

There were a couple bugs ...

First, we should just use the date of the two commits we're looking at rather
than looking at all the commits in between and picking the earliest one. This
can find _much_ earlier things that weren't merged until much later, which
really throws the numbers off.

Second, when calculating the number of weeks and months, we shouldn't use
POSIX::ceil(), that rounds up 4.01 weeks to 5. Instead, I wrote a simple
rounding function that does standard rounding.