Ruby 3 fixes

[anmolarora]

• One call to File.exists? was left by https://github.com/burke/zeus/pull/684, which is fixed in this PR.
• Tests pass on my local dev

  
  root@anmol-dev:~/go/src/github.com/burke/zeus# make test
  go mod download
  ZEUS_TEST_GEMPATH="/root/go/src/github.com/burke/zeus/rubygem" go test ./...
  ?       github.com/burke/zeus/go/clienthandler  [no test files]
  ?       github.com/burke/zeus/go/cmd/zeus       [no test files]
  ?       github.com/burke/zeus/go/config [no test files]
  ?       github.com/burke/zeus/go/processtree    [no test files]
  ?       github.com/burke/zeus/go/shinylog       [no test files]
  ?       github.com/burke/zeus/go/statuschart    [no test files]
  ?       github.com/burke/zeus/go/zerror [no test files]
  ?       github.com/burke/zeus/go/zeusclient     [no test files]
  ok      github.com/burke/zeus/go/filemonitor    (cached)
  ok      github.com/burke/zeus/go/messages       (cached)
  ok      github.com/burke/zeus/go/unixsocket     (cached)
  ?       github.com/burke/zeus/go/zeusversion    [no test files]
  ok      github.com/burke/zeus/go/zeusmaster     (cached)
  bundle -v || gem install bundler --no-document
  Bundler version 2.5.11
  cd rubygem && bundle check || bundle install
  The Gemfile's dependencies are satisfied
  cd rubygem && bin/rspec
  .............................

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