Require nightly rustfmt and use older nightly a bit

Author: ryoqun

ci/docker-rust-nightly/Dockerfile

@@ -4,6 +4,7 @@ ARG date
 RUN set -x \
  && rustup install nightly-$date \
  && rustup component add clippy --toolchain=nightly-$date \
+ && rustup component add rustfmt --toolchain=nightly-$date \
  && rustup show \
  && rustc --version \
  && cargo --version \

ci/rust-version.sh

@@ -24,7 +24,7 @@ fi
 if [[ -n $RUST_NIGHTLY_VERSION ]]; then
   nightly_version="$RUST_NIGHTLY_VERSION"
 else
-  nightly_version=2020-06-15
+  nightly_version=2020-06-10
 fi