Update to wildbit/postmark-php 2.x (#1)

Author: lozcalver

composer.json

@@ -11,7 +11,7 @@
 	"require": {
 		"php": ">=5.5",
 		"silverstripe/framework": "^3.1",
-		"wildbit/postmark-php": "^1.3"
+		"wildbit/postmark-php": "^2.5"
 	},
 	"extra": {
 		"installer-name": "postmark-mailer"