Update travis instructions

[mathiasbynens]

After running gem install travis, you need to run another step if you use travis-ci.com instead of travis-ci.org (which is being deprecated). The step is:

travis endpoint --pro --set-default

Background (no need to mention this in the docs): this updates the local config at ~/.travis/config.yml and ensures the travis CLI is uploading the environment secrets to the .com domain.

[alrra]

which is being deprecated

https://blog.travis-ci.com/2018-05-02-open-source-projects-on-travis-ci-com-with-github-apps

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After running gem install travis, you need to run another step if you use travis-ci.com instead of travis-ci.org (which is being deprecated). The step is:...

@mathiasbynens If the above is still true, do you want to open a PR? Thanks!

[mathiasbynens]

For some reason, the above command stopped working for me. I now have to explicitly use the --com flag, e.g. travis encrypt $file --> travis encrypt --com $file.