[5.1] Fixed support for PHP 7.1 (laravel#14549)

* Test on PHP 7.1

* Fixed cache incrementing on php 7.1

* Skipped the legacy encrypter test on PHP 7.1+

src/Illuminate/Cache/ArrayStore.php

@@ -50,7 +50,7 @@ public function put($key, $value, $minutes)
      */
     public function increment($key, $value = 1)
     {
-        $this->storage[$key] = $this->storage[$key] + $value;
+        $this->storage[$key] = ((int) $this->storage[$key]) + $value;
 
         return $this->storage[$key];
     }

src/Illuminate/Cache/DatabaseStore.php

@@ -165,7 +165,7 @@ protected function incrementOrDecrement($key, $value, Closure $callback)
             }
 
             $current = $this->encrypter->decrypt($cache->value);
-            $new = $callback($current, $value);
+            $new = $callback((int) $current, $value);
 
             if (! is_numeric($current)) {
                 return false;