Replace Travis with GitHub Actions

[variadico]

Currently, we use Travis CI for this repo. This change will move us to GitHub
Actions.

These are some of the things we need to do so we have parity with Travis.

• build
• pass tests
• upload coverage
• release*

goveralls doesn't support github actions.
https://docs.coveralls.io/supported-ci-services

codecov does though! https://docs.codecov.io/docs/supported-ci-providers

This PR switches this repo from coveralls to codecov for coverage reports.

*I need this PR to merge so I can test the release workflow.

This currently tests with Go 1.13 and 1.12.9. In addition, it tests with Linux,
macOS, and Windows.

[coveralls]

Coverage remained the same at 88.128% when pulling 2df7ac4 on github-actions into 21bd3c2 on master.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@e46a9d9). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #101   +/-   ##
=========================================
  Coverage          ?   86.24%           
=========================================
  Files             ?        6           
  Lines             ?      807           
  Branches          ?        0           
=========================================
  Hits              ?      696           
  Misses            ?       91           
  Partials          ?       20

Impacted Files	Coverage Δ
collector/collector.go	76.15% <100%> (ø)
collector/streaming.go	86.36% <100%> (ø)
exporter/exporter.go	89.6% <100%> (ø)
collector/replicator.go	86.79% <100%> (ø)

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[wallyqs]

LGTM!