Assert file exists

[mbernier]

Issue Summary

@mptap I am curious if you would be interested in switching these if statements over to test assertions? https://github.com/sendgrid/nodejs-http-client/blob/master/test/test.js

I really like how the tests you created in sendgrid/smtpapi-nodejs#60 ended up displaying in Travis. That would be amazing to have here too.

[mptap]

@mbernier: Sure thing, here you go! PR: #58

[mbernier]

Ohhhh yeah!! Thank you!