Add auto-deploy step on travis

[dhoeric]

Fixes #78

Checklist

• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the [Contribution Guide] and my PR follows them.
• [x] I updated my branch with the master branch.

Short description of what this PR does:

• Implement auto-deploy to rubygem

Need to put the RubyGem API key in TravisCI console with environment variable name RUBYGEM_API_KEY. It will auto-deploy to rubygem when a new tag is created.

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[SendGridDX]

All committers have signed the CLA.

[childish-sambino]

Duplicates https://github.com/sendgrid/ruby-http-client/pull/112