Update Documentation to reflect GCP python dependencies

[irivera007]

Description of issue:

This doc: https://github.com/Netflix/security_monkey/blob/develop/docs/update.md
states the required libraries/modules needed in order to pull data from GCP, however after installing the latest version I was unable to receive any data from GCP, while monitoring the workers I noticed some errors (which were marked as INFO for some reason) , these errors stated the lack of certain modules. Once I installed the modules , GCP data was flowing and I was getting emails about changes.

Extra python modules:
oauth2client
google-api-python-client

httplib2

Happy to create a PR if needed

[mikegrima]

Hello!

Yes, please submit a PR.