FIX: Restore FSVersion to string

[effigies]

#1958 changed FSVersion from a string to a LooseVersion object. This change corrects this issue.

[satra]

@effigies - was this breaking something, if so should there be a test for this to prevent future breakage?

[effigies]

Yes, see nipreps/niworkflows#151.

I'm actually not sure that anybody else would have depended on this, since it's something we added post 0.13.0-rc1, but I also don't want to pull the rug out from anybody who did. It didn't show up in our tests because we don't pin to master, but to specific merge commits. So it was found by a contributor who keeps his nipype bleeding edge.

Test passes locally.

[codecov-io]

Codecov Report

Merging #1962 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1962      +/-   ##
==========================================
+ Coverage   72.52%   72.52%   +<.01%     
==========================================
  Files        1070     1070              
  Lines       54382    54386       +4     
  Branches     7858     7858              
==========================================
+ Hits        39440    39444       +4     
  Misses      13715    13715              
  Partials     1227     1227

Flag	Coverage Δ
#smoketests	72.52% <100%> (ø)	⬆️
#unittests	70.07% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
...ype/interfaces/freesurfer/tests/test_preprocess.py	89.61% <100%> (+0.56%)	⬆️
nipype/interfaces/freesurfer/preprocess.py	66.7% <100%> (ø)	⬆️

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