Made libraries and bundles optional (#66)

You no longer need to specify at least one.

Fix: resolves #50

Dropped support for Ubuntu Xenial (#64)

Canonical have ended standard support for Ubuntu Xenial.

Replaced deprecated sha256sum key with checksum (#46)

Fixes deprecation warning:

```
TASK [gantsign.antigen : download Antigen] ***************************************************************
[DEPRECATION WARNING]: The parameter "sha256sum" has been deprecated and will be removed, use "checksum" 
instead. This feature will be removed from ansible-base in version 2.14. Deprecation warnings can be 
disabled by setting deprecation_warnings=False in ansible.cfg.
```

Increased minimum Ansible version to 2.5 (#19)

Ansible no longer supports version 2.4 and earlier.

Updated moleculew to version 0.9.9 (#16)

Keeping up with the latest changes.

Upgraded openSUSE version to 15.0 (#14)

Keeping up with the latest release.

Changed to allow environment variables in bundle URLs (#13)

The main use-case is to reference the `$HOME` of the current user.

Documented companion role (#8)

To help users who need to conditionally install bundles.

Renamed repository to follow new Ansible convention (#5)

Role names must now use underscores rather than dashes.