On TravisCI, use `sudo: required` to get Ubuntu 14.04
On TravisCI, use `sudo: required` to get Ubuntu 14.04

This change is motivated to obtain a C++ compiler that implements the
2011 ISO standard. `sudo` is not yet required.

tulip-control/tulip-control,tulip-control/tulip-control,tulip-control/tulip-control,tulip-control/tulip-control