Explicitly specify units for as.numeric.difftime

[jimhester]

Previously if the time was over a minute the default unit of auto would
convert the value to a decimal minute and you would get a display of
1.5s rather than 1m 30s.

[codecov-io]

Codecov Report

Merging #94 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #94   +/-   ##
=======================================
  Coverage   41.89%   41.89%           
=======================================
  Files          20       20           
  Lines         716      716           
=======================================
  Hits          300      300           
  Misses        416      416

Impacted Files	Coverage Δ
R/print-status.R	0% <0%> (ø)	⬆️

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[gaborcsardi]

Thanks!