Limit index name length to 63; psql NAMEDATALEN-1

Hi Jason, I ran into an issue with generated index names being too long, and found this solution proposed by Ward Vandewege to technoweenie in 2011. I tested the code in my own build (Rails 4 / Postgres 9), and it works well. 

Can you please pull it in?

Thanks again,
Martin

Ref: https://github.com/technoweenie/acts_as_versioned/issues/9