travis-ci: turn down strictness of performance test
travis-ci: turn down strictness of performance test

Previously, the linear performance assertion could fail on slower
machines (than a 2012 Macbook Pro), but not by much. I don't think we
need to assert four-nines of fit for the linear tests, three nines is
probably good enough (and one more than the default, btw).

woahdae/simple_xlsx_reader