Section 13.4: Predict runtime sec needs a revision

[pat-s]

Section "Strategize your high-performance computing": Some warnings in the code output and the plot shows a horizontal line only: https://books.ropensci.org/drake/time.html

[wlandau]

Thanks, Pat. The jobs argument of predict_runtime() changed to jobs_predict in ropensci/drake#1123. Now fixed. Travis should deploy it soon.