Put --allow-downgrades in the right place

.travis.yml

@@ -10,7 +10,7 @@ before_install:
   # list docker-engine versions
   - apt-cache madison docker-engine
   # upgrade docker-engine to specific version
-  - sudo apt-get -o Dpkg::Options::="--force-confnew" install -y --allow-downgrades docker-engine=${DOCKER_VERSION}
+  - sudo apt-get -o Dpkg::Options::="--force-confnew" --allow-downgrades install -y docker-engine=${DOCKER_VERSION}
   - docker version
   - docker info
   - sudo add-apt-repository ppa:duggan/bats --yes