multimedia/py-moviepy: Unbreak: it couldn't find the ffmpeg executable

This line:
from moviepy.editor import *

resulted in failure:

File "/usr/local/lib/python2.7/site-packages/imageio/plugins/ffmpeg.py", line 77, in download
  raise RuntimeError("FFMPEG exe isn't available for platform %s" % plat)

RuntimeError: FFMPEG exe isn't available for platform freebsd64

This is because it uses py-imageio to download the ffmpeg binary which doesn't exist for FreeBSD.
Besides, py-imageio is downloading files without checking cryptographic fingerprints wihich is insecure: https://github.com/imageio/imageio/issues/416

Solution:
Make py-moviepy to always use the pre-installed ffmpeg package.

Approved by: portmgr (unbreak)