Memoizes on_gce?

- also fixes an error in the gemspec.

googleauth.gemspec

@@ -5,7 +5,7 @@ require 'googleauth/version'
 
 Gem::Specification.new do |s|
   s.name          = 'googleauth'
-  s.version       = GoogleAuth::VERSION
+  s.version       = Google::Auth::VERSION
   s.authors       = ['Tim Emiola']
   s.email         = 'temiola@google.com'
   s.homepage      = 'https://github.com/google/google-auth-library-ruby'
@@ -13,7 +13,7 @@ Gem::Specification.new do |s|
   s.description   = <<-eos
    Allows simple authorization for accessing Google APIs.
    Provide support Application Default Credentials, as described at
-   https://developers.google.com/accounts/docs/application-default-credentials
+   https://developers.google.com/accounts/xdocs/application-default-credentials
   eos
 
   s.files         = `git ls-files`.split("\n")
@@ -27,6 +27,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'faraday', '~> 0.9'
   s.add_dependency 'logging', '~> 1.8'
   s.add_dependency 'jwt', '~> 1.2.1'
+  s.add_dependency 'memoist', '~> 0.11.0'
   s.add_dependency 'multi_json', '1.10.1'
   s.add_dependency 'signet', '~> 0.6.0'
 

lib/googleauth/compute_engine.rb

@@ -29,6 +29,7 @@
 
 require 'faraday'
 require 'googleauth/signet'
+require 'memoist'
 
 module Google
   # Module Auth provides classes that provide Google-specific authorization
@@ -43,16 +44,20 @@ module Google
                                'instance/service-accounts/default/token'
       COMPUTE_CHECK_URI = 'http://169.254.169.254'
 
-      # Detect if this appear to be a GCE instance, by checking if metadata
-      # is available
-      def self.on_gce?(options = {})
-        c = options[:connection] || Faraday.default_connection
-        resp = c.get(COMPUTE_CHECK_URI)
-        return false unless resp.status == 200
-        return false unless resp.headers.key?('Metadata-Flavor')
-        return resp.headers['Metadata-Flavor'] == 'Google'
-      rescue Faraday::ConnectionFailed
-        return false
+      class << self
+        extend Memoist
+        # Detect if this appear to be a GCE instance, by checking if metadata
+        # is available
+        def on_gce?(options = {})
+          c = options[:connection] || Faraday.default_connection
+          resp = c.get(COMPUTE_CHECK_URI)
+          return false unless resp.status == 200
+          return false unless resp.headers.key?('Metadata-Flavor')
+          return resp.headers['Metadata-Flavor'] == 'Google'
+        rescue Faraday::ConnectionFailed
+          return false
+        end
+        memoize :on_gce?
       end
 
       # Overrides the super class method to change how access tokens are