Force curl HTTP 1.1 headers according to PayPal requirements:
Starting October 7, 2013, we will require all incoming requests to have a “Host” header which complies with HTTP 1.1 Specifications. This header was not required under HTTP 1.0. IPN and PDT scripts using HTTP 1.0 may start failing with “HTTP/1.0 400 Bad Request” errors after October 7, 2013, which will result in IPN messages not being validated successfully, or PDT scripts not being able to retrieve transaction information.