title: Alter table modify enum in Knex js for Postgresql gives error
body: <p>I am using knex js and postgresql database. I have used a migration file to create a table <code>knex migrate:make create_car_table</code>. In this I have added a column fuel_type. <code>table.enu('fuel_type', ['PETROL', 'DIESEL', 'CNG'])</code>.</p>
  
  <p>Now I need to alter the table and I need these enum values <code>['HYBRID', 'ELECTRIC', 'PETROL', 'DIESEL']</code>.</p>
  
  <p>I have created another migration file using <code>knex migrate:make alter_car_table</code> and added the below code</p>
  
  <pre><code>exports.up = function(knex, Promise) {
      return knex.schema.alterTable('car', function (table) {
          table.enu('fuel_type', ['HYBRID', 'ELECTRIC', 'PETROL', 'DIESEL']).alter();
      });
  };
  
  exports.down = function(knex, Promise) {
      return knex.schema.alterTable('car', function (table) {
          table.enu('fuel_type', ['PETROL', 'DIESEL', 'CNG']).alter();
      });
  };
  </code></pre>
  
  <p>when I run <code>knex migrate:latest</code> I get the below error.</p>
  
  <pre><code>Knex:warning - migrations failed with error: alter table "car" alter column "fuel_type" type text check ("fuel_type" in ('HYBRID', 'ELECTRIC', 'PETROL', 'DIESEL')) using ("fuel_type"::text check ("fuel_type" in ('HYBRID', 'ELECTRIC', 'PETROL', 'DIESEL'))) - syntax error at or near "check"
  </code></pre>
  
  <p>I have refered <a href="http://knexjs.org/#Schema-alter" rel="noreferrer">Knex Js</a> for this.</p>