fix callback signature in convergence mcmcsample

[mileslucas]

#56 changed the callback signature but did not update it for the convergence sampling mcmcsample.

[codecov]

Codecov Report

Merging #66 (0a8ae72) into master (d961513) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #66   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files           6        6           
  Lines         187      187           
=======================================
  Hits          184      184           
  Misses          3        3           

Impacted Files	Coverage Δ
src/sample.jl	99.21% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d961513...0a8ae72. Read the comment docs.

[devmotion]

Can you update the version number (it should be a bugfix release) and add some tests?