Fix Travis warnings

[manastalukdar]

Build config validation
jobs.include.deploy: deprecated key skip_cleanup (not supported in dpl v2, use cleanup)
jobs.include.deploy: missing strategy, using the default git
root: missing os, using the default linux
root: missing language, using the default ruby
jobs.include.deploy: key github_token is an alias for token, using token

https://travis-ci.org/github/manastalukdar/learning-computer-science/builds/688253779/config