Bumped version of publishing plugin

[mockitoguy]

We need to use newer version of Gradle Plugin Portal publishing plugin. Otherwise Shipkit won't be able to publish to Gradle Plugin Portal. The error message from Travis CI:

This version of the com.gradle.plugin-publish plugin is no longer supported due to a critical security vulnerability. Please update to the latest version of the com.gradle.plugin-publish plugin (...)

[codecov]

Codecov Report

Merging #873 into master will increase coverage by 0.01%.
The diff coverage is 56.25%.

@@             Coverage Diff              @@
##             master     #873      +/-   ##
============================================
+ Coverage     76.38%   76.40%   +0.01%     
  Complexity     1235     1235              
============================================
  Files           221      221              
  Lines          4502     4505       +3     
  Branches        336      336              
============================================
+ Hits           3439     3442       +3     
  Misses          950      950              
  Partials        113      113              

Impacted Files	Coverage Δ	Complexity Δ
...contributors/github/GitHubContributorsFetcher.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ternal/notes/improvements/GitHubTicketFetcher.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...contributors/github/RecentContributorsFetcher.java	95.23% <100.00%> (ø)	8.00 <0.00> (ø)
...shipkit/internal/notes/util/GitHubListFetcher.java	71.11% <100.00%> (+1.34%)	10.00 <1.00> (ø)
...ipkit/internal/notes/util/GitHubObjectFetcher.java	91.30% <100.00%> (+0.39%)	5.00 <1.00> (ø)

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