Update HISTORY.md for 0.15.1

[shravanngoswamii]

Adds a 0.15.1 section to HISTORY.md covering the Hessian (order=2) work added in #163.

[github-actions]

AbstractPPL.jl documentation for PR #164 is available at:
https://TuringLang.github.io/AbstractPPL.jl/previews/PR164/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.59%. Comparing base (4390c43) to head (cffcb41).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #164   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files          15       15           
  Lines         982      982           
=======================================
  Hits          870      870           
  Misses        112      112           

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