Fixes database index names on postgres.

According to 4.1.1. Identifiers and Key Words [1] of the PostgreSQL
9.2.4 Documentation, NAMEDATALEN is 64 so the maximum identifier length
is 63 bytes.

[1] http://www.postgresql.org/docs/current/interactive/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
Merge pull request #167 from opf/fix/postgres

Fixes database index on postgres.