title: Sequelize Migration Error
body: <p>I'm using Sequelize 2.0.0-rc3 and there's an error I'm encountering; it seems like I'm writing the migration correctly but I'm getting an error trying to run it. I'm using Postgresql on the backend. Everything seems to be working fine; this is a new, isolated issue. The Document table and id column exists (created it in a previous migration, but discovered it's not auto-incrementing id's; so tried creating this migration to add auto-incrementing).</p>
  
  <pre><code>var p = require('bluebird');
  
  module.exports = {
      up: function (migration, DataTypes, done) {
          var promises = [];
          promises.push(
              migration.changeColumn(
                  'Document',
                  'id',
                  {
                      type: DataTypes.INTEGER,
                      primaryKey: true,
                      autoIncrement: true,
                      allowNull: false
                  }
              ));
          p.all(promises).then(done.bind(null, null)).catch(function (err) {
              console.error('Migration Failed: ', err);
              done(err);
          });
  
      },
  
      down: function (migration, DataTypes, done) {
          done();
      }
  };
  </code></pre>
  
  <blockquote>
    <p>Possibly unhandled TypeError: Cannot call method 'push' of undefined
        at Object.module.exports.QueryGenerator.dataTypeMapping (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/lib/dialects/postgres/query-generator.js:848:32)
        at Object.module.exports.QueryGenerator.pgDataTypeMapping (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/lib/dialects/postgres/query-generator.js:843:19)
        at Object.module.exports.QueryGenerator.changeColumnQuery (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/lib/dialects/postgres/query-generator.js:250:31)
        at module.exports.QueryInterface.changeColumn (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/lib/query-interface.js:345:37)
        at module.exports.Migration.(anonymous function) [as changeColumn] (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/lib/migration.js:26:50)
        at /Users/csimpson/code/temp-cause-server/database/migrations/20141216000001-alter-id-increment.js:8:41
        at tryCatch1 (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/node_modules/bluebird/js/main/util.js:45:21)
        at Promise$_callHandler [as _callHandler] (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/node_modules/bluebird/js/main/promise.js:660:13)
        at Promise$_settlePromiseFromHandler [as _settlePromiseFromHandler] (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/node_modules/bluebird/js/main/promise.js:675:18)
        at Promise$_settlePromiseAt (/Users/csimpson/code/temp-cause-server/node_modules/sequelize/node_modules/bluebird/js/main/promise.js:845:14)</p>
  </blockquote>