fix: find first associate pull request

[chingor13]

For some reason, a commit can have multiple associated pull requests. We actually want the first result, not the last and this appears to correctly be the associated pull request that we want.

Fixes #756

[codecov]

Codecov Report

Merging #764 (d8f3a6a) into master (07e2969) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #764      +/-   ##
==========================================
+ Coverage   90.02%   90.22%   +0.19%     
==========================================
  Files          57       57              
  Lines        7490     7467      -23     
  Branches      674      715      +41     
==========================================
- Hits         6743     6737       -6     
+ Misses        746      729      -17     
  Partials        1        1              

Impacted Files	Coverage Δ
src/github.ts	83.75% <100.00%> (ø)
src/bin/release-please.ts	88.38% <0.00%> (+6.48%)	⬆️

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[bcoe]

Let's see if this works.