Upgrade Fog dependency to version 1.25.0

Upgrade our dependency on Fog to versions 1.25.0 or above so as to
include the changes in this PR, which were released in 1.25.0:

    - [Preserve @ symbols in vcloud_director
      usernames](https://github.com/fog/fog/pull/3229)
Release version 0.15.0
Merge pull request #142 from gds-operations/upgrade_fog

Release version 0.15.0