`RuntimeError: No ffmpeg exe could be found` occured

I also faced #8 issue and solved it by executing the command below

```
!bash ./requirements.sh
```

But `RuntimeError: No ffmpeg exe could be found` occured.

```error.py
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)
in ()
4 from PIL import Image
5 import logging
----> 6 import pypianoroll
7 import scipy.stats
8 import pickle

~/anaconda3/envs/python3/lib/python3.6/site-packages/pypianoroll/__init__.py in ()
16
17 import pypianoroll.metrics
---> 18 from pypianoroll.multitrack import Multitrack
19 from pypianoroll.track import Track
20 from pypianoroll.utilities import (

~/anaconda3/envs/python3/lib/python3.6/site-packages/pypianoroll/multitrack.py in ()
11 from six import string_types
12
---> 13 from pypianoroll.track import Track
14 from pypianoroll.visualization import plot_multitrack
15

~/anaconda3/envs/python3/lib/python3.6/site-packages/pypianoroll/track.py in ()
7 from six import string_types
8
----> 9 from pypianoroll.visualization import plot_track
10
11

~/anaconda3/envs/python3/lib/python3.6/site-packages/pypianoroll/visualization.py in ()
15
16 try:
---> 17 from moviepy.editor import VideoClip
18 from moviepy.video.io.bindings import mplfig_to_npimage
19

~/anaconda3/envs/python3/lib/python3.6/site-packages/moviepy/editor.py in ()
34
35 # Clips
---> 36 from .video.io.VideoFileClip import VideoFileClip
37 from .video.io.ImageSequenceClip import ImageSequenceClip
38 from .video.io.downloader import download_webfile

~/anaconda3/envs/python3/lib/python3.6/site-packages/moviepy/video/io/VideoFileClip.py in ()
1 import os
2
----> 3 from moviepy.audio.io.AudioFileClip import AudioFileClip
4 from moviepy.Clip import Clip
5 from moviepy.video.io.ffmpeg_reader import FFMPEG_VideoReader

~/anaconda3/envs/python3/lib/python3.6/site-packages/moviepy/audio/io/AudioFileClip.py in ()
1 from __future__ import division
2
----> 3 from moviepy.audio.AudioClip import AudioClip
4 from moviepy.audio.io.readers import FFMPEG_AudioReader
5

~/anaconda3/envs/python3/lib/python3.6/site-packages/moviepy/audio/AudioClip.py in ()
5 from tqdm import tqdm
6
----> 7 from moviepy.audio.io.ffmpeg_audiowriter import ffmpeg_audiowrite
8 from moviepy.Clip import Clip
9 from moviepy.decorators import requires_duration

~/anaconda3/envs/python3/lib/python3.6/site-packages/moviepy/audio/io/ffmpeg_audiowriter.py in ()
5
6 from moviepy.compat import DEVNULL
----> 7 from moviepy.config import get_setting
8 from moviepy.decorators import requires_duration
9

~/anaconda3/envs/python3/lib/python3.6/site-packages/moviepy/config.py in ()
34 if FFMPEG_BINARY=='ffmpeg-imageio':
35 from imageio.plugins.ffmpeg import get_exe
---> 36 FFMPEG_BINARY = get_exe()
37
38 elif FFMPEG_BINARY=='auto-detect':

~/anaconda3/envs/python3/lib/python3.6/site-packages/imageio/plugins/ffmpeg.py in get_exe()
47 import imageio_ffmpeg
48
---> 49 return imageio_ffmpeg.get_ffmpeg_exe()
50
51

~/anaconda3/envs/python3/lib/python3.6/site-packages/imageio_ffmpeg/_utils.py in get_ffmpeg_exe()
48 # Nothing was found
49 raise RuntimeError(
---> 50 "No ffmpeg exe could be found. Install ffmpeg on your system, "
51 "or set the IMAGEIO_FFMPEG_EXE environment variable."
52 )

RuntimeError: No ffmpeg exe could be found. Install ffmpeg on your system, or set the IMAGEIO_FFMPEG_EXE environment variable.
```

I think the command such as `!conda install ffmpeg -y` should be included in `requrements.sh`