Remove `sudo: required` from `.travis.yml` (#37)

Summary:
This is just https://github.com/facebook/bistro/pull/37/, backported to `fbcode_builder`.

snarkmaster
Pull Request resolved: https://github.com/facebook/bistro/pull/37

Test Plan: Push and watch Travis

Reviewed By: simpkins

Differential Revision: D16453080

Pulled By: snarkmaster

fbshipit-source-id: a15eaead931f046c41e50f8e3b412ef68b172d65