switch back to previous libreoffice mirror site

 * OSU only keeps the most recent release, causing our image builds to
   fail after a patch version release.  Switch back to
   documentfoundation.org's mirror which keeps historical releases.

Author: felliott

Dockerfile

@@ -46,9 +46,9 @@ RUN usermod -d /home www-data \
     && pip install setuptools==37.0.0 \
     && mkdir -p /code
 
-ENV LIBREOFFICE_VERSION 6.0.2
+ENV LIBREOFFICE_VERSION 6.0.2.1
 ENV LIBREOFFICE_ARCHIVE LibreOffice_6.0.2.1_Linux_x86-64_deb.tar.gz
-ENV LIBREOFFICE_MIRROR_URL https://ftp.osuosl.org/pub/tdf/libreoffice/testing/
+ENV LIBREOFFICE_MIRROR_URL https://downloadarchive.documentfoundation.org/libreoffice/old/
 RUN apt-get update \
     && apt-get install -y \
         curl \