Add builds:cache:purge command

[dmathieu]

Using the new public endpoint instead of a one-off dyno.

See #52.
Merging this won't let us close that issue yet though, as there's still heroku/heroku-repo/issues/70 which is currently being discussed.

[codecov-io]

Codecov Report

Merging #54 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   96.61%   96.74%   +0.13%     
==========================================
  Files           8        9       +1     
  Lines         118      123       +5     
==========================================
+ Hits          114      119       +5     
  Misses          4        4

Impacted Files	Coverage Δ
index.js	100% <ø> (ø)	⬆️
commands/builds/cache-purge.js	100% <100%> (ø)

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