Log Message:
p5-Parallel-ForkManager: update to 1.20.

1.20 2018-07-18
  [ DOCUMENTATION ]
    - Add section about rand() and forked processes. (GH#22)

  [ ENHANCEMENTS ]
    - Expand abbreviations in error messages. (GH#14, Ricardo Signes)
    - Rename variables and simplify `_waitpid` signature. (GH#14, Ricardo
      Signes)

  [ STATISTICS ]
    - code churn: 6 files changed, 80 insertions(+), 33 deletions(-)