Ignore `k` at the end of fps numbers from ffmpeg

I had problems reading an mp3 file with moviepy. When moviepy tried to find out
the tbr, ffmpeg sent `90k`, which couldn't be converted to a float. I made it
ignore the `k` in `moviepy/video/io/ffmpeg_reader.py`. I don't know what the `k`
means, but it seems to work for me.