[videobam] Set age_limit to 18

From [their ToS](http://videobam.com/terms): "User must be eighteen 18[sic] years of age or older to use or access this web site."

youtube_dl/extractor/videobam.py

@@ -76,4 +76,5 @@ def _real_extract(self, url):
             'uploader': uploader,
             'view_count': view_count,
             'formats': formats,
+            'age_limit': 18,
         }