2.3/fix doctrine dependency

This resolves the following issues.

## Missing require
* Doctrine\DBAL is not specified as a require in the illuminate\database repo.
https://github.com/illuminate/database/blob/master/composer.json
* Illuminate\Database\Schema\Builder depends on Doctrine\DBAL\Schema\MySqlSchemaManager
* So I added it to our composer.json

## Fix “unknown database type enum requested"
 * Should probably be addressed in Doctrine, if not in Illuminate\Capsule
 * Doctrine 2: Resolving “unknown database type enum requested” - http://wildlyinaccurate.com/doctrine-2-resolving-unknown-database-type-enum-requested
 * Unknown enum database type in migration file / Doctrine issue - https://github.com/laravel/framework/issues/1346
 * Error when mapping database enum field type - https://github.com/symfony/symfony/issues/866