Setting Test environment variables in rails without putting in source code

I'm using Twilio for an app and on production I set the auth token using heroku's CLI. I'm using sms-spec (http://ift.tt/1IwO14q) to test my app's Twilio integration locally. I want to set ENV['TWILIO_AUTH_TOKEN'] to my token in the test environment.

I use guard to auto-run my tests whenever I make changes so I don't want to have to manually set the ENV variable each time I run tests. I also don't want to put the token in my source code for security reasons.

Is there a way I can set the ENV variable for my local test environment such that it is permanent and not in my source? I've spent a few hours researching this and can't seem to find a good explanation of how to do this. Any help is much appreciated :)