Update guzzle version to ~3.8
Update guzzle version to ~3.8

Allows using the library together with other libraries that require
guzzle 3.9

AlexeyKupershtokh/store-receipt-validator,stanimir-kukudov/store-receipt-validator,stanimir-kukudov/store-receipt-validator,cpascal/store-receipt-validator,skonb/store-receipt-validator,aporat/store-receipt-validator