flux-cron: coerce values to integer when using %d

Problem: On centos8 some of the tests in t0016-cron-faketime.t
fail with the following error:

 lua: /usr/src/src/cmd/flux-cron:102:
   bad argument flux-framework#2 to 'fmt' (number has no integer representation)

Coerce values to integer using math.floor when %d is used in string.format
to avoid this error.

src/cmd/flux-cron

@@ -78,7 +78,7 @@ local function reladate (t)
         return fmt ("a minute ago")
     end
     if (diff < 90) then
-        return fmt ("%d minutes ago", diff)
+        return fmt ("%d minutes ago", math.floor (diff + 0.5))
     end
 
     -- Convert to hours:
@@ -99,7 +99,7 @@ local function reladate (t)
         return fmt ("%d weeks ago", (d + 3) / 7)
     end
     if (d < 365) then
-        return fmt ("%d months ago", (d + 15) / 30)
+        return fmt ("%d months ago", math.floor((d + 15) / 30))
     end
 
     return fmt ("%.1f years ago", diff / 365)