<?php namespace Kyslik\ColumnSortable;

use Illuminate\Support\ServiceProvider;

class ColumnSortableServiceProvider extends ServiceProvider {

    /**
     * Indicates if loading of the provider is deferred.
     *
     * @var bool
     */
    protected $defer = false;

    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        $this->publishes([__DIR__ . '/../../config/columnsortable.php' => config_path('columnsortable.php')]);
        $this->mergeConfigFrom(__DIR__ . '/../../config/columnsortable.php', 'columnsortable');
        $this->registerBladeExtensions();
    }

    /**
     * Register the service provider.
     *
     * @return void
     */
    public function register()
    {
        //
    }

    /**
     * Register Blade extensions.
     *
     * @return void
     */
    protected function registerBladeExtensions()
    {

        $blade = $this->app['view']->getEngineResolver()->resolve('blade')->getCompiler();

        $blade->extend(function ($view, $compiler)
        {
            $pattern = $compiler->createMatcher('sortablelink');

            $replace = '<?php echo \Kyslik\ColumnSortable\Sortable::link(array $2);?>';

            return preg_replace($pattern, $replace, $view);
        });

    }

}

<?php namespace Kyslik\ColumnSortable;

use Illuminate\Support\ServiceProvider;

class ColumnSortableServiceProvider extends ServiceProvider {

    /**
     * Indicates if loading of the provider is deferred.
     *
     * @var bool
     */
    protected $defer = false;

    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        $this->publishes([__DIR__ . '/../../config/columnsortable.php' => config_path('columnsortable.php')]);
        $this->registerBladeExtensions();
    }

    /**
     * Register the service provider.
     *
     * @return void
     */
    public function register()
    {
        $this->mergeConfigFrom(__DIR__ . '/../../config/columnsortable.php', 'columnsortable');
    }

    /**
     * Register Blade extensions.
     *
     * @return void
     */
    protected function registerBladeExtensions()
    {

        $blade = $this->app['view']->getEngineResolver()->resolve('blade')->getCompiler();

        $blade->extend(function ($view, $compiler)
        {
            $pattern = $compiler->createMatcher('sortablelink');

            $replace = '<?php echo \Kyslik\ColumnSortable\Sortable::link(array $2);?>';

            return preg_replace($pattern, $replace, $view);
        });

    }

}

Move mergeConfigFrom to register method
Move mergeConfigFrom to register method

According to http://laravel.com/docs/5.0/packages#configuration it should be in register() and not in boot()