handle missing settings on Guzzle BearerAuth plugin events

See https://www.flowdock.com/app/imagineeasy/easybib-dev/messages/1961493 et seq.

this doesn't work with PHP 5.4 (https://travis-ci.org/easybiblabs/oauth2-client-php/jobs/37185685) ;)

(I think it's best to just drop the 5.4 build)