Fix gradle wrapper and CONTRIBUTING.md #41

[kobylynskyi]

Fixes #41

[codecov]

Codecov Report

Merging #42 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #42   +/-   ##
=========================================
  Coverage     88.85%   88.85%           
  Complexity      262      262           
=========================================
  Files            32       32           
  Lines           610      610           
  Branches         83       83           
=========================================
  Hits            542      542           
  Misses           45       45           
  Partials         23       23           

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