Update override for supported_extension_aliases

The attribute name changed to include a _ prefix.

Change-Id: I8948f550ee7672806f63f60dcc7f128c5b5b406f
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge pull request #13 from dhellmann/bug/aliases-typeerror-dhc-1042

Update override for supported_extension_aliases