Add unittest to check for specific repo files

[thinkingserious]

This repo should have the following list of files included:

• ./Docker or docker/Docker
• ./docker-compose.yml or ./docker/docker-compose.yml
• ./.env_sample
• ./.gitignore
• ./.travis.yml
• ./.codeclimate.yml
• ./CHANGELOG.md
• ./CODE_OF_CONDUCT.md
• ./CONTRIBUTING.md
• ./.github/ISSUE_TEMPLATE
• ./LICENSE.md
• ./.github/PULL_REQUEST_TEMPLATE
• ./README.md
• ./TROUBLESHOOTING.md
• ./USAGE.md
• ./USE_CASES.md

This PR is only asking for tests of the existence of these files, if the files do not exist when you run the tests - do not worry about the tests not passing. We will identify this and create a new PR for the issue.
Thank you!

[luan-cestari]

hi @thinkingserious I saw this issue opened in several projects, I will create a PR fo each one of them like I did in sendgrid/nodejs-http-client#52, ok?

[mptap]

@mbernier: PR: #54