Explicitly set docker backend

Explicitly set it to vfs since apparently !29 (merged) was insufficient to fix #2 (closed).

x86_64-linux-centos7/Dockerfile

@@ -11,10 +11,6 @@ RUN yum -y install coreutils binutils which git make \
     ncurses-compat-libs gmp-devel ncurses-devel gcc-c++ findutils \
     curl wget jq
 
-# Python 3
-RUN yum install -y https://centos7.iuscommunity.org/ius-release.rpm; \
-    yum install -y python34u python34u-libs
-
 # Documentation tools
 RUN yum -y install python-sphinx \
     texlive texlive-latex texlive-xetex \