Since it appears that this was last updated 6 months ago, and at the same time that @sgerrand appears to have left Duffel for another position.
Combined with errors in the main documentation page, and lacking functionality in the various classes (IE: Airports class, all() takes a parameter array so you could pass parameters to it for finding other than the 'first 50 results'. But it then ignores that parameter).
Should it be assumed that Duffel has abandoned direct support of PHP developers with this library?