Updating Travis encryption file

To update the Travis encryption file, which contains things like the Salesforce credentials/Mapbox token/Accounts Oauth tokens for testing, follow the below steps.

Install Travis CLI and login

$ gem install travis

$ travis login --org

Encrypt file, move to the correct location, and update .travis.yml

Start by updating the needed variables

$ travis encrypt-file openstax/settings/local.py

This spits out directions on the line you need to update in .travis.yml.

Note: make sure you move the file to openstax/settings/local.py.enc and update the path in .travis.yml