Revert "centos-7: GCC 8 -> 9"

CUDA 10.1 does not support GCC 9+
This reverts commit a12fed62.

centos-7/Dockerfile

@@ -8,7 +8,7 @@ LABEL authors="Moritz Maxeiner <moritz.maxeiner@fu-berlin.de>"
 ENV BASH_ENV "/etc/profile"
 RUN yum install -y centos-release-scl && \
     yum install -y epel-release && \
-    yum install -y devtoolset-9 && \
+    yum install -y devtoolset-8 && \
     yum install -y rh-git218-git && \
     yum clean all
 ADD bash/toolchain.sh /etc/profile.d/toolchain.sh

centos-7/bash/toolchain.sh

-# GCC 9
-source scl_source enable devtoolset-9
+# GCC 8
+source scl_source enable devtoolset-8
 
 # Git 2.18
 source scl_source enable rh-git218