Avoid using sudo in Travis-CI runs.

[glennpratt]

Using sudo in Travis-CI runs means a project can't opt-in to using container based workers, which may be faster / more available.

http://docs.travis-ci.com/user/workers/container-based-infrastructure/

rust-ci/tpt/ppatrigger/templates/ppatrigger/put_artifacts_script.txt

Line 19 in 3712b9e

sudo apt-get install python-virtualenv > /dev/null

I believe Travis-CI has their own S3 tool already installed, but even if that doesn't work there is probably another tool that could be installed without sudo.