use explicit bundler version (#3353)

To resolve issue with https://app.travis-ci.com/github/travis-infrastructure/k8s-deploy/builds/268346424#L859-L861

Author: qasim-at-tci

Dockerfile

@@ -24,7 +24,7 @@ WORKDIR /app
 COPY Gemfile      /app
 COPY Gemfile.lock /app
 
-RUN gem install bundler
+RUN gem install bundler:2.4.22
 RUN bundler install --verbose --retry=3
 RUN gem install --user-install executable-hooks