chore: Update travis - add codecov

[pushkyn]

Fixes #74

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Update travis - add codecov

If you have questions, please send an email to Sendgrid, or file a Github Issue in this repository.

[thinkingserious]

Hello @pushkyn,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX