fix(changelog): generate today's date when using an unreleased_version

[woile]

No description provided.

[codecov]

Codecov Report

Merging #182 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #182   +/-   ##
=======================================
  Coverage   91.76%   91.76%           
=======================================
  Files          35       35           
  Lines         947      947           
=======================================
  Hits          869      869           
  Misses         78       78           

Flag	Coverage Δ
#unittests	91.76% <100.00%> (ø)

Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <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 75fe50f...2fe9c74. Read the comment docs.