.circleci/outer-build.sh: update comment; PPC build now uses Rust 1.36

The dep ppv-lite86 started using a construct that was only stabilized in Rust
1.32, and our previous bigendian build image only used 1.31.1.

Note that the name of the downloaded bigendian build image file hasn't
changed, so that this commit doesn't actually change anything in the code; it
might be best to give new build images new filenames. But, I need a commit to
give the CI an excuse to rerun, so here we go.

.circleci/outer-build.sh

@@ -1,5 +1,5 @@
 #! /bin/bash
-# Copyright 2018 the Tectonic Project
+# Copyright 2018-2019 the Tectonic Project
 # Licensed under the MIT License
 
 set -e -x
@@ -10,7 +10,7 @@ buildroot=$HOME
 # that can be run via QEMU. This tarball was generated according to the
 # procedure describe here:
 # https://github.com/tectonic-typesetting/tectonic-ci-support/blob/master/bigendian/README.md .
-# The current version embeds Rust 1.31.1.
+# The current version embeds Rust 1.36.0.
 
 tarball='https://github.com/tectonic-typesetting/tectonic-ci-support/releases/download/production/tectonic-buildenv-ppc.tar.gz'