Fix check for wkhtmltopdf binary.

[ADmad]

Refs #304

[codecov]

Codecov Report

Merging #308 (6fc6c71) into master (21c0de1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #308   +/-   ##
=========================================
  Coverage     52.02%   52.02%           
  Complexity      212      212           
=========================================
  Files            10       10           
  Lines           544      544           
=========================================
  Hits            283      283           
  Misses          261      261           

Impacted Files	Coverage Δ	Complexity Δ
src/Pdf/Engine/WkHtmlToPdfEngine.php	69.23% <100.00%> (ø)	32.00 <0.00> (ø)

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