Replace exit 0 with set -e

This may have been causing Travis to always pass.
Remove the install_core_packages function

We no longer need to install curl, openssl and ca-certificates because
they have been added to the default LXC template.

Travis also has these installed by default.
Use TRAVIS_BUILD_DIR for the roles_path

The old roles_path did not work because Travis writes to
$HOME/build/$TRAVIS_REPO_SLUG, not $HOME/$TRAVIS_REPO_SLUG.

However Travis also exposes $TRAVIS_BUILD_DIR which is the absolute path
to what we want, so let's use it.
Remove the backport sources on Travis

We only need to do this on non-Travis hosts. This is a huge win too
because we no longer have to run apt-get update on Travis, it should
speed up tests by quite a bit.
Update the example .travis.yml config

The virtualenv property lets you use system packages instead venv.

The before_install/install properties is another performance win, it
informs Travis to skip running those hooks.
Update the changelog for August 31st 2014
Merge pull request #4 from nickjj/moreupdates

Prevent Travis from always exiting with 0 and adjust a few Travis settings