Fix version compare

[stevepiercy]

This PR fixes the following error message.

TASK [Fail if Ansible is old] *****************************************************************************************************************************************************************************************************************************************
fatal: [example.com]: FAILED! => {"msg": "The conditional check 'ansible_version is version('2.5.0', 'lt')' failed. The error was: Version comparison: '<' not supported between instances of 'str' and 'int'\n\nThe error appears to be in '/project-path/plone/ansible-playbook/playbook.yml': line 11, column 7, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n    - name: Fail if Ansible is old\n      ^ here\n"}

Its complement was merged in plone/ansible.plone_server#134