Fix undefined variable error on PHP 5.6

[hkdobrev]

I have experienced a PHP error on this line: https://github.com/jjaffeux/postmark-inbound-php/blob/1cdc9b2d620487a6fbb1171eca7e07fb2fb1e386/lib/Postmark/Inbound.php#L89 about the $header variable being undefined.

This has happened on PHP 5.6 with the Postmark webhook check request:

{"FromName":"Postmarkapp Support","From":"support@postmarkapp.com","FromFull":{"Email":"support@postmarkapp.com","Name":"Postmarkapp Support","MailboxHash":""},"To":"\"Firstname Lastname\" <mailbox+SampleHash@inbound.postmarkapp.com>","ToFull":[{"Email":"mailbox+SampleHash@inbound.postmarkapp.com","Name":"Firstname Lastname","MailboxHash":"SampleHash"}],"Cc":"\"First Cc\" <firstcc@postmarkapp.com>, secondCc@postmarkapp.com","CcFull":[{"Email":"firstcc@postmarkapp.com","Name":"First Cc","MailboxHash":""},{"Email":"secondCc@postmarkapp.com","Name":"","MailboxHash":""}],"Bcc":"\"First Bcc\" <firstbcc@postmarkapp.com>, secondbcc@postmarkapp.com","BccFull":[{"Email":"firstbcc@postmarkapp.com","Name":"First Bcc","MailboxHash":""},{"Email":"secondbcc@postmarkapp.com","Name":"","MailboxHash":""}],"OriginalRecipient":"mailbox+SampleHash@inbound.postmarkapp.com","Subject":"Test subject","MessageID":"73e6d360-66eb-11e1-8e72-a8904824019b","ReplyTo":"replyto@postmarkapp.com","MailboxHash":"SampleHash","Date":"Mon, 21 Sep 2015 08:17:50 -0400","TextBody":"This is a test text body.","HtmlBody":"&lt;html&gt;&lt;body&gt;&lt;p&gt;This is a test html body.&lt;\/p&gt;&lt;\/body&gt;&lt;\/html&gt;","StrippedTextReply":"This is the reply text","Tag":"TestTag","Headers":[{"Name":"X-Header-Test","Value":""}],"Attachments":[{"Name":"test.txt","Content":"VGhpcyBpcyBhdHRhY2htZW50IGNvbnRlbnRzLCBiYXNlLTY0IGVuY29kZWQu","ContentType":"text\/plain","ContentLength":45}]}

While I am executing:

$inbound = new \Postmark\Inbound($json);
$inbound->Headers('X-Spam-Status');

[jjaffeux]

I haven't been doing any php for years, I will trust you on this, merge. Thanks.

[jjaffeux]

@hkdobrev I have also pushed a new major version to packagist dropping support of old php, thanks again.

[hkdobrev]

Thanks! :beers: