add VumiBackend to __init__ so rapidsms.backends.vumi.VumiBackend works
add VumiBackend to __init__ so rapidsms.backends.vumi.VumiBackend works

peterayeni/rapidsms,lsgunth/rapidsms,peterayeni/rapidsms,caktus/rapidsms,ehealthafrica-ci/rapidsms,lsgunth/rapidsms,lsgunth/rapidsms,eHealthAfrica/rapidsms,catalpainternational/rapidsms,ehealthafrica-ci/rapidsms,peterayeni/rapidsms,catalpainternational/rapidsms,peterayeni/rapidsms,ehealthafrica-ci/rapidsms,eHealthAfrica/rapidsms,lsgunth/rapidsms,caktus/rapidsms,caktus/rapidsms,catalpainternational/rapidsms,catalpainternational/rapidsms,eHealthAfrica/rapidsms