Add missing dep

Missing File::Slurp:

https://rt.cpan.org/Public/Bug/Display.html?id=96297

Also note that File::Slurp currently has issues:

https://rt.perl.org/Public/Bug/Display.html?id=121870
Merge pull request #2 from MartinMcGrath/patch-1

Add missing dependency. The test needs it. Thanks Martin McGrath!