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