fix: Update travis - add codecov

[pushkyn]

Fixes #51

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:

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3766c77). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #52   +/-   ##
=========================================
  Coverage          ?   99.18%           
=========================================
  Files             ?        2           
  Lines             ?      122           
  Branches          ?        0           
=========================================
  Hits              ?      121           
  Misses            ?        1           
  Partials          ?        0

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[hugovk]

I've tested this and it still fixes the CI:

https://travis-ci.org/hugovk/smtpapi-python/builds/436143424

Current master is erroring:

https://travis-ci.org/hugovk/smtpapi-python/builds/435803743

[thinkingserious]

Hello @pushkyn,

Thanks again for the PR!

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

Team SendGrid DX