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.
```

tasks/install.yml

@@ -19,7 +19,7 @@
   get_url:
     url: '{{ antigen_redis_mirror }}/{{ antigen_redis_filename }}'
     dest: '{{ antigen_download_dir }}/{{ antigen_local_filename }}'
-    sha256sum: '{{ antigen_redis_sha256sum }}'
+    checksum: 'sha256:{{ antigen_redis_sha256sum }}'
     mode: 'u=rw,go=r'
 
 - name: create install directory