[7.x] Fix CURRENT_TIMESTAMP precision bug

[jgusta]

This fixes a bug using the MySQL schema grammar. When defining a migration, setting a fractional second precision in a timestamp field in MySQL would fail if the useCurrent() modifier is used.

Example in a migration:

        Schema::create('failed_jobs', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->text('connection');
            $table->text('queue');
            $table->longText('payload');
            $table->longText('exception');
            $table->timestamp('failed_at', 6)->useCurrent();
        });

When migrating produces an error:

SQLSTATE[42000]: Syntax error or access violation: 1067 Invalid default value for 'failed_at' (SQL: create table `failed_jobs` (`id` bigint unsigned not null auto_increment primary key, `connection` text not null, `queue` text not null, `payload` longtext not null, `exception` longtext not null, `failed_at` timestamp(6) default CURRENT_TIMESTAMP not null) default character set utf8mb4 collate 'utf8mb4_unicode_ci' engine = InnoDB ROW_FORMAT=DYNAMIC)

This fix makes sure the precision modifier is added to CURRENT_TIMESTAMP default, if specified.

[GrahamCampbell]

Could you add a test case please?