When floats are returned as binaries from the database driver, convert them accordingly.

I'm not 100% sure whether this is *the* correct approach, but at least the epgsql driver seems to convert floats to binaries, and when ::float() is specified for a given model attribute, that will be returned as a binary from the driver causing a function_clause error like e.g.

    exit with reason no function clause matching boss_record_lib:convert_value_to_type(<<"29.80">>, float) line 78

This approach at least seems to work, but if there are better ways of solving this, I won't object. ;-)