See failure.

Chrome 56.0.2924 (Linux 0.0.0) amp-form Var Substitution should substitute hidden fields variables in XHR async FAILED
AssertionError: expected 'amp--MEn6bEkgSVHw56rzwQKXztL5XRitMBLRMtRo8AfaXaLvbk-4MHRae1AE7WG_ukx' to match /amp-\w+/
at /tmp/688b7d6108a484955606c3168b6acfba.browserify:57087:42 <- /home/travis/build/ampproject/amphtml/extensions/amp-form/0.1/test/test-amp-form.js:1075:41

Seems like that regex needs to be made slightly more liberal.