fixed bug with negative seconds when round is set to false
fixed bug with negative seconds when round is set to false

Bug description : 5:50 expected instead of 6:-10
If round option is set to false, readingTimeMinutes need to be
calculated with Math.floor.

michael-lynch/reading-time,JustinBeaudry/reading-time,kevinrodbe/reading-time,kevinrodbe/reading-time