Allow init to accept a list with single element

Having trouble using raven_lager_backend in our dev/prod environments.
Hoping this resolves the crash (below):

Lager failed to install handler raven_lager_backend into lager_event, retrying later : {'EXIT',
                                                                                        {undef,
                                                                                         [{raven_lager_backend,
                                                                                           init,
                                                                                           [info],
                                                                                           []},