Fix deprecated read and write

[devmotion]

Fixes a bug introducing when deprecating read and write. Instead of overloading Base.read and Base.write we accidentally added new MCMCChains.read and MCMCChains.write methods.

[codecov]

Codecov Report

Merging #241 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #241   +/-   ##
=======================================
  Coverage   72.54%   72.54%           
=======================================
  Files          20       20           
  Lines        1242     1242           
=======================================
  Hits          901      901           
  Misses        341      341           

Impacted Files	Coverage Δ
src/MCMCChains.jl	66.66% <ø> (ø)

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[yebai]

Thanks, @devmotion -- Looks good to me!