chore(slugbuilder): revert 41fc96a in Dockerfile

We have this storage bucket now and can go back to the old way of
publishing object-storage-cli rather than hosting a binary in the git
repo (the Deis maintainers released the bucket, I grabbed it.)

rootfs/Dockerfile

@@ -30,8 +30,8 @@ RUN sed -i -e 's/^deb-src/#deb-src/' /etc/apt/sources.list && \
 
 ADD . /
 ENV PYTHONPATH $PYTHONPATH:/usr/local/lib/python3/site-packages
-RUN mv objstorage /bin/objstorage && \
-    chmod +x /bin/objstorage && \
+ADD https://storage.googleapis.com/object-storage-cli/bb8e054/objstorage-bb8e054-linux-amd64 /bin/objstorage
+RUN chmod +x /bin/objstorage && \
     chown -R slug:slug /app && \
     chown slug:slug /bin/get_object \
                     /bin/normalize_storage \