added rb mode to google api client

I'm a Ruby newbie so I don't fully understand this issue, but thought I would issue this pull request in case it helps. When I tried to use the original gem, I received an 
```
ArgumentError: Invalid keyfile or passphrase
``` 
error. The two articles below helped me trace my issue to the way the Google APIClient was working. I made the following change and it worked for me. I am on a Mac OS 10.7.5, using Ruby 2.0, Rails 4.

http://blog.afry.de/2012/12/ruby-meets-google-calendar-api-or-ruby.html
http://stackoverflow.com/questions/10202254/why-openssl-on-windows-produces-error-but-not-on-centos-pkcs12-parse-mac-verif