Remove support/testing for Ruby 1.8

This version of Ruby doesn't maintain the order of hashes which makes it
non-trivial to render the config hash in a way that doesn't vary on every
run. We're not using Ruby 1.8 and hopefully less other people are now. So
remove it.
Merge pull request #5 from alphagov/remove_ruby18_support

Remove support/testing for Ruby 1.8
package_ensure param to override version

Allow the package version to be overridden so that an existing machine can
be upgraded to a newer version of Aptly.