api break 0.7 -> 0.8, raw binding are not optional

[jfgodoy]

if the second argument in knex.raw(sql, [bindings]) is not setted, when two raws are merged, the undefined value is added as a binding value. see the next example:

knex.raw('select * from ? where id=?', [knex.raw('foo'), 4]).toSQL()
{ method: 'raw',
  sql: 'select * from foo where id=?',
  bindings: [ undefined, 4 ],
  options: {} }