Update from pow() -> **.

[SpacePossum]

With the bump to PHP7 we can start modernizing the code (this was added in PHP 5.6)

[codecov-io]

Codecov Report

Merging #49 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #49   +/-   ##
=========================================
  Coverage     99.29%   99.29%           
  Complexity      108      108           
=========================================
  Files             7        7           
  Lines           282      282           
=========================================
  Hits            280      280           
  Misses            2        2

Impacted Files	Coverage Δ	Complexity Δ
src/Differ.php	98.48% <100%> (ø)	54 <0> (ø)	⬇️

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[sebastianbergmann]

Thanks!