Ruby 3 fixes

[anmolarora]

• One call to File.exists? was left by Replacing calls to the alias File.exists? (deprecated since Ruby v3.x) with File.exist? #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
.............................

Finished in 0.03815 seconds (files took 0.12246 seconds to load)
29 examples, 0 failures