Bad hostname when production environment is selected

[alnacle]

Moving to production implies passing the host variable to the SDK. The SDK does not change the internal hostname correctly.

The following code does not work:

amadeus = Amadeus(client_id: "foo",
                   client_secret: "bar",
                   environment: ["hostname": "production"])

Two issues here:

• hostname should be host instead. README file must be updated accordingly.
• the internal host must point to api.amadeus.com.