With Cmdstan 2.22+ use JSON by default

[rok-cesnovar]

Reverts #100 since the underlying cmdstan issue will be resolved for 2.22

Closes #109

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@4845369). Click here to learn what that means.
The diff coverage is 63.63%.

@@            Coverage Diff            @@
##             master     #110   +/-   ##
=========================================
  Coverage          ?   92.49%           
=========================================
  Files             ?        7           
  Lines             ?     1186           
  Branches          ?        0           
=========================================
  Hits              ?     1097           
  Misses            ?       89           
  Partials          ?        0

Impacted Files	Coverage Δ
R/model.R	97.05% <60%> (ø)
R/args.R	98.8% <66.66%> (ø)
R/path.R	95.65% <66.66%> (ø)

---

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[rok-cesnovar]

This is ready for review. Codecov on appveyor seems to not be working, so I decided to comment it out until we sort out the reason.

[jgabry]

Thanks, looks good! Just one addition requested.

[rok-cesnovar]

Appveyor tests pass but for some reason the codecov tests fail. I am not sure why.

[jgabry]

I don't know why codecov is failing either, but this looks good. I'll merge it now.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.34%. Comparing base (44e1d5c) to head (856285e).
⚠️ Report is 2898 commits behind head on master.

Files with missing lines	Patch %	Lines
R/model.R	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
- Coverage   92.64%   92.34%   -0.31%     
==========================================
  Files           7        7              
  Lines        1183     1188       +5     
==========================================
+ Hits         1096     1097       +1     
- Misses         87       91       +4     

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