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

.travis.yml

 # Facebook projects that use `fbcode_builder` for continuous integration
 # share this Travis configuration to run builds via Docker.
 
-sudo: required
-
 # Docker disables IPv6 in containers by default. Enable it for unit tests that need [::1].
 before_script:
   - if [[ "$TRAVIS_OS_NAME" != "osx" ]];