Update Google-API-Client.podspec.json

Update Google-API-Client.podspec.json for [revision 416](https://code.google.com/p/google-api-objectivec-client/source/list) - the latest version as of August 19, 2014; the current version of this spec is for revision 393.
This update adds the [Gmail API](https://code.google.com/p/google-api-objectivec-client/source/browse/#svn%2Ftrunk%2FSource%2FServices%2FGmail%2FGenerated) to the list of subspecs