[ci] Move CI to GitHub Actions (fixes #215)

[jameslamb]

This PR attempts to fix #215. See that issue and the links in it for details. Basically, Travis is becoming a lot harder for open source projects to use, and I think GitHub Actions is mature enough that we can move this project to it.

[codecov-io]

Codecov Report

Merging #217 (0564dae) into master (8bcd886) will increase coverage by 2.34%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   93.13%   95.47%   +2.34%     
==========================================
  Files           8        8              
  Lines         597      597              
==========================================
+ Hits          556      570      +14     
+ Misses         41       27      -14     

Impacted Files	Coverage Δ
R/es_search.R	90.90% <0.00%> (+3.03%)	⬆️
R/get_fields.R	100.00% <0.00%> (+5.21%)	⬆️

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[austin3dickey]

cute

[jameslamb]

thank you, thank you

[austin3dickey]

testing whether the transient-failed build will block merge even if I approve

[austin3dickey]

Yep LGTM! Looks like you just need to rebuild because it timed out trying to pull data.table from CRAN in one of the tests. Great work!

[jameslamb]

Looks like you just need to rebuild because it timed out trying to pull data.table from CRAN in one of the tests

😬 hopefully that's just a fluke. If it becomes a real problem we could probably change the timeout or start caching those packages or something

[jameslamb]

I just changed the branch rules to require that all GitHub Actions jobs pass before a PR can be merged