Updated RethinkDB install instructions

[jvwong]

Their PGP key location has changed so the build will fail.

See new instructions: https://rethinkdb.com/docs/install/ubuntu/

[jvwong]

RUN \
  echo "deb https://download.rethinkdb.com/repository/ubuntu-`lsb_release -c -s` `lsb_release -c -s` main" | tee /etc/apt/sources.list.d/rethinkdb.list && \
  wget -qO- https://download.rethinkdb.com/repository/raw/pubkey.gpg | apt-key add - && \
  apt-get update  && \
  apt-get install -y rethinkdb