Manage fixed version changes

Currently, if we have:

```
maven { "/the/file":
    ensure     => latest,
    groupid    => "the.group.id",
    artifactid => "artifact",
    version    => "1.0",
}
```

and change it to:
```
maven { "/the/file":
    ensure     => latest,
    groupid    => "the.group.id",
    artifactid => "artifact",
    version    => "1.1",
}
```

...nothing happens, because the outdated? method returns true only if the updatable? returns true. But in this case, we do not want the artifact to be updated, we change the target version.

A workaround was to delete /the/file and run puppet again.

This patch tries to get the artifact stored in the local repo (with "-o"). If the artifact is missing or different from /the/file, it is outdated (not the right version).