Update environment.yml

The same issue (PILLOW_VERSION import error) as #180 when running the first import cell in training notebooks on my VM.
Updated environment.yml by adding an older version of Pillow for the conda environment to fix this.
Everything works fine now.

environment.yml

@@ -11,6 +11,7 @@ dependencies:
 - tensorboardX=1.6
 - youtube-dl>=2019.4.17
 - jupyterlab
+- pillow=6.2.1
 - pip:
   - ffmpeg-python==0.1.17
   - opencv-python>=3.3.0.10