Update doc to include the use of travis gem to encrypt the repo_token

[Chris Allison]

Ref. https://docs.travis-ci.com/user/code-climate/

[devon blandin]

Hi @quimarche,

We don't consider the Code Climate repo token secret. It is used only to submit test coverage data and can be safely committed directly into a public CI configuration file. This token is not authorized to access any data within a Code Climate account or its repositories.

We have a longer answer here on our docs site.

You're free to encrypt the value if you wish but we don't consider it a required step.