Remove a hard dependency on the intl extension (#289)

Fixes #288.

While #268 and #269 updated count() and isCountable() to now work with intl resources, #286 made that the whole beberlei/assert package now requires the intl extension to be installed.

However, the intl extension dependency is a soft dependency.
The package does not need intl for most of its features and can safely work without it.

composer.json

@@ -40,7 +40,6 @@
   },
   "require": {
     "php": "^7",
-    "ext-intl": "*",
     "ext-simplexml": "*",
     "ext-mbstring": "*",
     "ext-ctype": "*",

tests/Assert/Tests/AssertTest.php

@@ -1437,7 +1437,8 @@ public function testValidCount()
      * @requires extension intl
      */
     public function testValidCountWithIntlResourceBundle()
-    {        // Test ResourceBundle counting using resources generated for PHP testing of ResourceBundle
+    {
+        // Test ResourceBundle counting using resources generated for PHP testing of ResourceBundle
         // https://github.com/php/php-src/commit/8f4337f2551e28d98290752e9ca99fc7f87d93b5
         $this->assertTrue(Assertion::count(new ResourceBundle('en_US', __DIR__.'/_files/ResourceBundle'), 6));
     }