getBasesResponses should specify 'Content-Type:application/json' in curl options

[Akm0d]

The function getBasesResponses() in /src/models/Level.php should specify that it is expecting to receive json data in the curl options. This gives greater flexibility to the server hosting the base challenge.

Basically, the following line of code needs to be added to line 1136 of /src/models/Level.php:
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));

[mptap]

@jtylers: Please could you review? #597

[justinwray]

@mptap

Your PR looked great. You will just need to resubmit to the dev branch.

[mptap]

@justinwray: Done! #610