Updated Migrator to use correct timestamps

In response to https://github.com/antonioribeiro/tracker/issues/7

===================================================================

I am getting

```
Call to undefined method PragmaRX\Tracker\Support\Database\Migrator::timestamp()
```

and I think it could be to do with the fact that you have

```php
$this->timestamp('created_at')->index();
$this->timestamp('updated_at')->index();
```

everywhere instead of 

```php
$table->timestamp('created_at')->index();
$table->timestamp('updated_at')->index();
```

Full ERROR:

```
Migration create_tracker_tables created successfully!
Generating optimized class loader
Running for workbench [fdw/core]...
Running for workbench [fdw/newsletter]...
Running for workbench [fdw/pages]...
Running for workbench [fdw/blog]...
Running for workbench [fdw/cart]...
PHP Fatal error:  Call to undefined method PragmaRX\Tracker\Support\Database\Migrator::timestamp() in /media/bravo/Development/website/bitbucket/website/vendor/pragmarx/tracker/src/Support/Database/Migrator.php on line 61
PHP Stack trace:
PHP   1. {main}() /media/bravo/Development/website/bitbucket/website/artisan:0
PHP   2. Symfony\Component\Console\Application->run() /media/bravo/Development/website/bitbucket/website/artisan:59
PHP   3. Symfony\Component\Console\Application->doRun() /media/bravo/Development/website/bitbucket/website/vendor/symfony/console/Symfony/Component/Console/Application.php:121
PHP   4. Symfony\Component\Console\Application->doRunCommand() /media/bravo/Development/website/bitbucket/website/vendor/symfony/console/Symfony/Component/Console/Application.php:191
PHP   5. Illuminate\Console\Command->run() /media/bravo/Development/website/bitbucket/website/vendor/symfony/console/Symfony/Component/Console/Application.php:885
PHP   6. Symfony\Component\Console\Command\Command->run() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Console/Command.php:96
PHP   7. Illuminate\Console\Command->execute() /media/bravo/Development/website/bitbucket/website/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:241
PHP   8. Fdw\Core\Console\SetupCommand->fire() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Console/Command.php:108
PHP   9. Fdw\Core\Console\SetupCommand->fireOtherPackages() /media/bravo/Development/website/bitbucket/website/workbench/fdw/core/src/Fdw/Core/Console/SetupCommand.php:34
PHP  10. Fdw\Core\Console\SetupCommand->fireTrackerPackage() /media/bravo/Development/website/bitbucket/website/workbench/fdw/core/src/Fdw/Core/Console/SetupCommand.php:80
PHP  11. Illuminate\Console\Command->call() /media/bravo/Development/website/bitbucket/website/workbench/fdw/core/src/Fdw/Core/Console/SetupCommand.php:91
PHP  12. Illuminate\Console\Command->run() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Console/Command.php:124
PHP  13. Symfony\Component\Console\Command\Command->run() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Console/Command.php:96
PHP  14. Illuminate\Console\Command->execute() /media/bravo/Development/website/bitbucket/website/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:241
PHP  15. Illuminate\Database\Console\Migrations\MigrateCommand->fire() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Console/Command.php:108
PHP  16. Illuminate\Database\Migrations\Migrator->run() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Database/Console/Migrations/MigrateCommand.php:65
PHP  17. Illuminate\Database\Migrations\Migrator->runMigrationList() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Database/Migrations/Migrator.php:80
PHP  18. Illuminate\Database\Migrations\Migrator->runUp() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Database/Migrations/Migrator.php:109
PHP  19. CreateTrackerTables->up() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Database/Migrations/Migrator.php:133
PHP  20. PragmaRX\Tracker\Support\Database\Migrator->up() /media/bravo/Development/website/bitbucket/website/app/database/migrations/2014_06_11_040853_create_tracker_tables.php:16
PHP  21. PragmaRX\Tracker\Support\Database\Migrator->execute() /media/bravo/Development/website/bitbucket/website/vendor/pragmarx/tracker/src/Support/Database/Migrator.php:41
PHP  22. PragmaRX\Tracker\Support\Database\Migrator->createTables() /media/bravo/Development/website/bitbucket/website/vendor/pragmarx/tracker/src/Support/Database/Migrator.php:402
PHP  23. Illuminate\Database\Schema\Builder->create() /media/bravo/Development/website/bitbucket/website/vendor/pragmarx/tracker/src/Support/Database/Migrator.php:63
PHP  24. PragmaRX\Tracker\Support\Database\Migrator->PragmaRX\Tracker\Support\Database\{closure}() /media/bravo/Development/website/bitbucket/website/vendor/laravel/framework/src/Illuminate/Database/Schema/Builder.php:110
{"error":{"type":"Symfony\\Component\\Debug\\Exception\\FatalErrorException","message":"Call to undefined method PragmaRX\\Tracker\\Support\\Database\\Migrator::timestamp()","file":"\/media\/bravo\/Development\/website\/bitbucket\/website\/vendor\/pragmarx\/tracker\/src\/Support\/Database\/Migrator.php","line":61}}
```