Fixed model validation erroneously triggering on non-nullable text fields

refs https://github.com/TryGhost/Toolbox/issues/309

- I've just ran into a problem when deleting the `defaultTo` field on a
  non-nullable `text` column in our schema because this validation
  thinks there should be a value set
- `text` fields cannot have defaults so the schema is incorrect, and the
  validation triggering is a bug that's preventing it from being
  cleaned up
- the default is defined on the model so I don't think we're losing
  anything here

core/server/data/schema/validator.js

@@ -45,6 +45,8 @@ function validateSchema(tableName, model, options) {
         // check nullable
         if (Object.prototype.hasOwnProperty.call(schema[tableName][columnKey], 'nullable') &&
             schema[tableName][columnKey].nullable !== true &&
+            Object.prototype.hasOwnProperty.call(schema[tableName][columnKey], 'type') &&
+            schema[tableName][columnKey].type !== 'text' &&
             !Object.prototype.hasOwnProperty.call(schema[tableName][columnKey], 'defaultTo')
         ) {
             if (validator.isEmpty(strVal)) {