name of a sequence name must be a usual DB meta-identifier: 31 chars maximum in Firebird

Fix seq name limit in firebird driver 
http://stackoverflow.com/questions/22096198/rails-jaybird-set-seq-name-for-generator

The name of a generator must be a usual DB meta-identifier: 31 chars maximum
http://www.firebirdsql.org/manual/generatorguide-sqlsyntax.html