Update to latest version of go-github

[lkysow]

Now that google/go-github#693 is merged, we can use the go-github code to validate webhook requests that use secrets.

We still need to parse the request ourselves if they're not using secrets since the go-github library doesn't have that capability.

[codecov-io]

Codecov Report

Merging #157 into master will increase coverage by 0.63%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
+ Coverage   43.61%   44.24%   +0.63%     
==========================================
  Files          16       16              
  Lines        1261     1243      -18     
==========================================
  Hits          550      550              
+ Misses        690      672      -18     
  Partials       21       21

Impacted Files	Coverage Δ
server/server.go	0% <0%> (ø)	⬆️

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