Use Travis legacy VMs instead of containers

This allows installation of pip and virtualenv which s3-tests
requires.

.travis.yml

@@ -1,8 +1,6 @@
-sudo: false
+sudo: required
 language: java
-addons:
-  apt_packages:
-   - python-pip
-   - python-virtualenv
-   - libevent-dev
+before_install:
+ - sudo apt-get update -qq
+ - sudo apt-get install -qq python-pip python-virtualenv libevent-dev
 after_script: src/test/resources/run-s3-tests.sh