PHPBB2: Fixed mapping for ConversationMessage

ConversationId was mapped to group_id, but group_id doesn't exist in the source table, it's groupid instead. In the effect after the import there is no connection between conversations and messages (user see list of conversations but they are all empty), this fixes that.
Merge pull request #28 from tgarbiak/patch-1

PHPBB2: Fixed mapping for ConversationMessage