[ticket/14972] Bump PHP requirements to 5.6. Update dependencies.

Also bump to PHP 7.2 compatible versions: PHPUnit to 5.7, Guzzle to 6.3.

PHPBB3-14972

phpBB/composer.json

@@ -25,10 +25,10 @@
 		"phpbb/phpbb-core": "self.version"
 	},
 	"require": {
-		"php": ">=5.5.9",
+		"php": ">=5.6",
 		"bantu/ini-get-wrapper": "1.0.*",
 		"google/recaptcha": "~1.1",
-		"guzzlehttp/guzzle": "~6.2",
+		"guzzlehttp/guzzle": "~6.3",
 		"lusitanian/oauth": "^0.8.1",
 		"marc1706/fast-image-size": "1.1.*",
 		"paragonie/random_compat": "^1.2",
@@ -55,7 +55,7 @@
 		"laravel/homestead": "~4.0",
 		"phing/phing": "2.4.*",
 		"phpunit/dbunit": "~2.0",
-		"phpunit/phpunit": "~4.8",
+		"phpunit/phpunit": "^5.7",
 		"squizlabs/php_codesniffer": "2.*",
 		"symfony/browser-kit": "~3.1",
 		"symfony/css-selector": "~3.1",
@@ -68,7 +68,7 @@
 	},
 	"config": {
 		"platform": {
-			"php": "5.5.9"
+			"php": "5.6"
 		}
 	}
 }