message: add -y for apt commands
diff: BLOCKSTACK_BRANCH="rc-0.14.2"
  # get bitcoind
  -sudo add-apt-repository ppa:bitcoin/bitcoin || exit 1
  -sudo apt-get update || exit 1
  -sudo apt-get install bitcoin || exit 1
  +sudo add-apt-repository -y ppa:bitcoin/bitcoin || exit 1
  +sudo apt-get -y update || exit 1
  +sudo apt-get -y install bitcoin || exit 1
  # needed on CircleCI's VMs
  pip install --upgrade pip
  