Fix warning: File.exists? is deprecated
Fix warning: File.exists? is deprecated

File.exist? should be used instead.
Might consider using File.file? instead of File.exist? since File.exist? will
return true when given the path of a file or a directory.

Addresses the following warning:

lib/google/api_client/service/simple_file_store.rb:127: warning: File.exists? is a deprecated name, use File.exist? instead

landonwilkins/google-api-ruby-client,jonahglover/google-api-ruby-client,barancw/google-api-ruby-client,hunzinker/google-api-ruby-client,jamox/google-api-ruby-client,googleapis/google-api-ruby-client,hunzinker/google-api-ruby-client,jmburges/google-api-ruby-client,googleapis/google-api-ruby-client,saicheems/google-api-ruby-client,saicheems/google-api-ruby-client,MaxGabriel/google-api-ruby-client,jmburges/google-api-ruby-client,MaxGabriel/google-api-ruby-client,cocktail-io/google-api-ruby-client,austin-wang/google-api-ruby-client,rusikf/google-api-ruby-client,remi/google-api-ruby-client,hanpanpan200/google-api-ruby-client,sugitak/google-api-ruby-client,andihalimi3/ruby-newb,keylimetoolbox/google-api-ruby-client,keylimetoolbox/google-api-ruby-client,austin-wang/google-api-ruby-client,landonwilkins/google-api-ruby-client,sugitak/google-api-ruby-client,sqrrrl/google-api-ruby-client,andihalimi3/ruby-newb,remi/google-api-ruby-client,hanpanpan200/google-api-ruby-client,sqrrrl/google-api-ruby-client,jamox/google-api-ruby-client,barancw/google-api-ruby-client,jonahglover/google-api-ruby-client,cocktail-io/google-api-ruby-client,rusikf/google-api-ruby-client