vcs/github_client: add logging around github API calls

[cket]

This PR adds debug level logging to indicate when Atlantis is making github API calls. This will be helpful when we approach GH rate limits and want some visibility into when + why calls are made.

[codecov]

Codecov Report

Merging #1042 into master will increase coverage by 0.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #1042      +/-   ##
==========================================
+ Coverage   71.96%   71.98%   +0.01%     
==========================================
  Files          65       65              
  Lines        5429     5436       +7     
==========================================
+ Hits         3907     3913       +6     
- Misses       1215     1216       +1     
  Partials      307      307              

Impacted Files	Coverage Δ
server/server.go	63.77% <0.00%> (ø)
server/events/vcs/github_client.go	80.89% <87.50%> (+0.19%)	⬆️

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[lkysow]

🎉