Fix MultiScene save_animation when distributed isn't installed

[djhoese]

If distributed is not installed and the user doesn't explicitly ask for client=False then it still tried to use distributed and would fail. This fixes it so the client is not used if distributed is not installed.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[coveralls]

Coverage increased (+0.07%) to 78.732% when pulling e31dfc3 on djhoese:bugfix-multiscene-client into 0e5236f on pytroll:master.

[codecov]

Codecov Report

Merging #677 into master will increase coverage by 0.75%.
The diff coverage is 95.91%.

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
+ Coverage   78.66%   79.41%   +0.75%     
==========================================
  Files         138      138              
  Lines       20357    21048     +691     
==========================================
+ Hits        16013    16716     +703     
+ Misses       4344     4332      -12

Impacted Files	Coverage Δ
satpy/tests/test_multiscene.py	97.84% <100%> (+0.36%)	⬆️
satpy/multiscene.py	89.13% <77.77%> (+2.04%)	⬆️
satpy/tests/test_scene.py	99.43% <0%> (-0.05%)	⬇️
satpy/readers/__init__.py	95.65% <0%> (+1.1%)	⬆️
satpy/scene.py	93.18% <0%> (+3.67%)	⬆️

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[djhoese]

It was also brought up by @ColinDuff that imageio>2.5.0 may not work on all platforms or in all situations. This will need more investigation but downgrading for now is a good enough solution. There are no explicit tests in satpy that actually call in to imageio (we mock it).

@ColinDuff would you mind filing an issue with the error messages that you were getting (that is, the errors after passing client=False)?