Raised PHP requirement to 7.1 and updated types.

Fixes #25

src/Provider/Bootable_Provider_Interface.php

@@ -1,9 +1,13 @@
 <?php
+declare( strict_types=1 );
 
 namespace Rarst\Laps\Provider;
 
 use Rarst\Laps\Plugin;
 
+/**
+ * Interface for providers that need to boot up once plugin is ready to start.
+ */
 interface Bootable_Provider_Interface {
 
 	/**
@@ -11,5 +15,5 @@ interface Bootable_Provider_Interface {
 	 *
 	 * @return void
 	 */
-	public function boot( Plugin $laps );
+	public function boot( Plugin $laps ): void;
 }