.travls.yml: use official cache settings for gradle

[whym]

https://docs.travis-ci.com/user/languages/java/#Projects-Using-Gradle

[codecov-io]

Codecov Report

Merging #852 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #852   +/-   ##
=====================================
  Coverage     6.9%   6.9%           
=====================================
  Files          95     95           
  Lines        5070   5070           
  Branches      472    472           
=====================================
  Hits          350    350           
  Misses       4693   4693           
  Partials       27     27

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