COMPAT: Use cholesky for cholfact

[oyamad]

To have MVNSampler work in v0.7 by avoiding RankDeficientException (used in Games.jl).

[codecov-io]

Codecov Report

Merging #214 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
+ Coverage   90.93%   90.93%   +<.01%     
==========================================
  Files          25       25              
  Lines        1776     1777       +1     
==========================================
+ Hits         1615     1616       +1     
  Misses        161      161

Impacted Files	Coverage Δ
src/sampler.jl	96.42% <100%> (+0.13%)	⬆️

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[sglyon]

Thank you!