Update PMD to 6.0.1

[vanniktech]

Currently 'blocked' by pmd/pmd#869

[codecov]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #21   +/-   ##
======================================
  Coverage    63.9%   63.9%           
======================================
  Files           5       5           
  Lines         133     133           
  Branches       19      19           
======================================
  Hits           85      85           
  Misses         43      43           
  Partials        5       5

Impacted Files	Coverage Δ
.../com/vanniktech/rxpermission/MockRxPermission.java	100% <ø> (ø)	⬆️

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