Update helpers.R -> pinvgamma used directly

[twolodzko]

This hack is not needed any more as pinvgamma was fixed. Next time if you find such bug, feel free to post issue on my github page (https://github.com/twolodzko/extraDistr/).

[codecov]

Codecov Report

Merging #106 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #106   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          21     21           
  Lines        2422   2422           
=====================================
  Hits         2422   2422

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[goldingn]

Great, thanks!
I certainly will let you know - I had forgotten about that hack.

(p.s. I've changed the target of this PR to the dev branch since I'm using master for intermediate releases only)