Use Aardvark as the default format for indexing records

[thatbudakguy]

This changes the indexer so that when you do:

rake geocombine:index

It prefers Aardvark format records by default (instead of 1.0), since that's now the current standard. You can still do:

SCHEMA_VERSION='1.0' rake geocombine:index

To index v1.0 instead.

It also updates the README to add this info (and a little bit of other info).