Update gemfile for auto devops rack app

What does this MR do?

We are currently seeing transient issues when building images in the build phase while deploying through Auto Devops. Example:

79 ---> 87b91dc103e0
80Step 7/8 : RUN bundle install
81 ---> Running in 77d768145104
82Warning: the running version of Bundler (1.17.2) is older than the version that created the lockfile (1.17.3). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
83Fetching gem metadata from https://rubygems.org/..
84Retrying dependency api due to error (2/4): Bundler::HTTPError Network error while fetching https://index.rubygems.org/api/v1/dependencies?gems=rack%2Crake (too many connection resets (due to Net::ReadTimeout with #<TCPSocket:(closed)> - Net::ReadTimeout) after 0 requests on 47429367136420, last used 1596536091.4369624 seconds ago)..
85Fetching rake 12.3.3
86Retrying download gem from https://rubygems.org/ due to error (2/4): Gem::RemoteFetcher::UnknownHostError timed out (https://rubygems.org/gems/rake-12.3.3.gem)
87Retrying download gem from https://rubygems.org/ due to error (3/4): Gem::RemoteFetcher::UnknownHostError timed out (https://rubygems.org/gems/rake-12.3.3.gem)Installing rake 12.3.3
88Using bundler 1.17.2

Causing tests to fail: #221283 (comment 390245210)

a possible solution could be to update the Gemfile.