Timeout after 2.5min

Use the --timeout flag to change the timeout.

Reproduce

herbie shell --seed 2020273 
(FPCore (x n)
  :name "2nthrt (problem 3.4.6)"
  :precision binary64
  (- (pow (+ x 1.0) (/ 1.0 n)) (pow x (/ 1.0 n))))