Fix incompatible gem versions in DOCKER=centos CI

* gem update --system tries to use rubygems-update 3.0 which requires
  ruby >= 2.3.0, so use ~> 2.7
* latest bundler 2.x also requires ruby >= 2.3.0, so use ~> 1.17

Author: felixbuenemann

.travis_Dockerfile_centos

@@ -17,7 +17,7 @@ RUN yum -y install \
   gcc \
   gcc-c++ \
   make
-RUN gem update --system > /dev/null
-RUN gem install bundler
+RUN gem install --no-document "rubygems-update:~>2.7" && update_rubygems
+RUN gem install --no-document "bundler:~>1.17"
 
 CMD sh .travis_centos.sh