twitter/secureheaders — pull request #44 by wmakley (2013-03-29 15:17:15+00:00)
Title: Fix failing example config setting in Readme: config.hsts = {:max_age => 99 [...]

Not sure why the gem worked for me before actually, but hopefully this is a clean enough fix to merge. The Readme has an integer value for max_age, which the StrictTransportSecurity class was throwing an error for.