Fix command fb_version_generate when VERSIONS_BASEDIR is set

When VERSIONS_BASEDIR is set, fb_version_generate ignores that settings and generate thumbnail next to original photos, because version_generator function requires path parameter to be relative to MEDIA_ROOT, but in current version absolute path is passed in and therefore os.path.join(VERSIONS_BASEDIR, path) ignores the first parameter (second one is absolute path).