Coerce max_connections to an integer

max_connections must be a non-zero integer. It so happens that calling
to_i on things that aren't integers returns 0, which should fail
validation correctly.

Otherwise, if you specify the max_connections attribute as an integer,
the existing (convoluted) check fails because you can't to_i a Fixnum.
Merge pull request #176 from schrodinger/max_connections

Coerce max_connections to an integer