Replace deprecated method that cause ruby warnings File.exit?

lib/google/api_client/auth/file_storage.rb

@@ -42,7 +42,7 @@ module Google
       ##
       # Attempt to read in credentials from the specified file.
       def load_credentials
-        if File.exist? self.path
+        if File.exists? self.path
           File.open(self.path, 'r') do |file|
             cached_credentials = JSON.load(file)
             @authorization = Signet::OAuth2::Client.new(cached_credentials)

lib/google/api_client/client_secrets.rb

@@ -63,7 +63,7 @@ module Google
         end
         while filename == nil
           search_path ||= File.expand_path('.')
-          if File.exist?(File.join(search_path, 'client_secrets.json'))
+          if File.exists?(File.join(search_path, 'client_secrets.json'))
             filename = File.join(search_path, 'client_secrets.json')
           elsif search_path == '/' || search_path =~ /[a-zA-Z]:[\/\\]/
             raise ArgumentError,

lib/google/api_client/service.rb

@@ -120,7 +120,7 @@ module Google
         if options.include? :cache_store
           @cache_store = options[:cache_store]
         else
-          cache_exists = File.exist?(DEFAULT_CACHE_FILE)
+          cache_exists = File.exists?(DEFAULT_CACHE_FILE)
           if (cache_exists && File.writable?(DEFAULT_CACHE_FILE)) ||
              (!cache_exists && File.writable?(Dir.pwd))
             @cache_store = Google::APIClient::Service::SimpleFileStore.new(