Remove the need to use File::Slurp in Makefile.PL. Using File::Slurp in ...

Remove the need to use File::Slurp in Makefile.PL. Using File::Slurp in Makefile.PL creates a dependency problem because it is used before the dependencies are solved.

I have had trouble with installing Git::Hooks using plenv, local directory perl and carton. I have Git::Hooks in my cpanfile. Git::Hooks uses File::Slurp in customized Makefile.PL but File::Slurp is not part of Perl Core Modules. To go around the situation, I have had to first install File::Slurp into (plenv) system wide Perl, and only then Carton manages to install Git::Hooks locally (in my local directory's subdirectory 'local'. In our setup, it is important that all Git::Hooks related modules stay in local directory.