What version of the twilio library did you use? It's a problem with underscore:
jweis at MacBook-Pro in ~/mysrc/weisjohn/tessel-code on master*
$ tr twilio.js
TESSEL! Connected to TM-00-04-f0009a30-0067433c-404445c2.
INFO Bundling directory /Users/jweis/mysrc/weisjohn/tessel-code
INFO Deploying bundle (3.64 MB)...
INFO Running script...
...ode_modules/twilio/node_modules/underscore/underscore.js:898: attempt to call method 'propertyIsEnumerable' (a nil value)