roll latest, devel back to default cdn, maybe better?

r-ver/Dockerfile

@@ -12,10 +12,7 @@ ENV R_VERSION=${R_VERSION:-3.4.1} \
     LANG=en_US.UTF-8 \
     TERM=xterm
 
-## dependencies. Use cloudfront, official CDN gives frequent 503 errors
-RUN sed -i "s/deb.debian.org/cdn-fastly.deb.debian.org/" /etc/apt/sources.list \
-  && sed -i "s/security.debian.org/cdn-fastly.debian.org\/debian-security/" /etc/apt/sources.list \
-  && apt-get update \
+RUN apt-get update \
   && apt-get install -y --no-install-recommends \
     bash-completion \
     ca-certificates \

r-ver/devel/Dockerfile

@@ -13,9 +13,7 @@ ENV R_VERSION=${R_VERSION:-3.4.1} \
     TERM=xterm
 
 ## dependencies. Use cloudfront, official CDN gives frequent 503 errors
-RUN sed -i "s/deb.debian.org/cloudfront.debian.net/" /etc/apt/sources.list \
-  && sed -i "s/security.debian.org/cloudfront.debian.net\/debian-security/" /etc/apt/sources.list \
-  && apt-get update \
+RUN apt-get update \
   && apt-get install -y --no-install-recommends \
     bash-completion \
     ca-certificates \