Remove pegged Bundler version

Author: jywarren

Dockerfile

@@ -36,11 +36,10 @@ RUN wget https://github.com/webnicer/chrome-downloads/raw/master/x64.deb/google-
     mv chromedriver /usr/local/bin/chromedriver && \
     chmod +x /usr/local/bin/chromedriver
 
-ENV BUNDLER_VERSION=2.1.4
 WORKDIR /tmp
 ADD Gemfile /tmp/Gemfile
 ADD Gemfile.lock /tmp/Gemfile.lock
-RUN gem install --default bundler && \
+RUN bundle update --bundler && \
     gem update --system && \
     bundle install --jobs=4