OSBS.get_build_logs(): use BuildResponse for state parsing.
OSBS.get_build_logs(): use BuildResponse for state parsing.

Could do with more mock JSON responses for better test coverage,
especially errored/cancelled builds.

From line note in #66:
https://github.com/DBuildService/osbs/pull/66/files#r30439416

TomasTomecek/osbs,pombredanne/osbs-client,DBuildService/osbs-client,bfontecc007/osbs-client,projectatomic/osbs-client,jpopelka/osbs-client,pombredanne/osbs-client,DBuildService/osbs-client,pbabinca/osbs,bfontecc007/osbs-client,projectatomic/osbs-client,jpopelka/osbs-client,TomasTomecek/osbs,twaugh/osbs-client,vrutkovs/osbs-client,vrutkovs/osbs-client,twaugh/osbs-client,pbabinca/osbs