Update version and changelog for 0.8.0 release (#188)

lib/googleauth/version.rb

@@ -31,6 +31,6 @@ module Google
   # Module Auth provides classes that provide Google-specific authorization
   # used to access Google APIs.
   module Auth
-    VERSION = '0.7.1'.freeze
+    VERSION = '0.8.0'.freeze
   end
 end