gh: Update to use latest kerl

This also solves the issue that a 27 release candidate was selected
as a backward compat testing release.

.github/dockerfiles/Dockerfile.ubuntu-base

@@ -47,17 +47,19 @@ RUN mkdir /buildroot /tests /otp && chown ${USER}:${GROUP} /buildroot /tests /ot
 
 ## We install the latest version of the previous three releases in order to do
 ## backwards compatability testing of Erlang.
-RUN apt-get install -y git curl && \
-    curl -L https://raw.githubusercontent.com/kerl/kerl/3.1.0/kerl > /usr/bin/kerl && \
+RUN apt-get update && apt-get install -y git curl && \
+    curl -L https://raw.githubusercontent.com/kerl/kerl/master/kerl > /usr/bin/kerl && \
     chmod +x /usr/bin/kerl && \
     kerl update releases && \
-    LATEST=$(kerl list releases | tail -1 | awk -F '.' '{print $1}') && \
+    LATEST=$(kerl list releases | grep "\*$" | tail -1 | awk -F '.' '{print $1}') && \
     for release in $(seq $(( LATEST - 2 )) $(( LATEST ))); do \
-      VSN=$(kerl list releases | grep "^$release" | tail -1); \
+      VSN=$(kerl list releases | grep "^$release" | tail -1 | awk '{print $1}'); \
       if [ $release = $LATEST ]; then \
         echo "/usr/local/lib/erlang-${VSN}/bin" > /home/${USER}/LATEST; \
       fi && \
+      echo "Building ${VSN}" && \
       kerl build ${VSN} ${VSN} && \
+      echo "Installing ${VSN} to /usr/local/lib/erlang-${VSN}" && \
       kerl install ${VSN} /usr/local/lib/erlang-${VSN}; \
     done && \
     rm -rf ~/.kerl