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.

Ref: https://github.com/gnustavo/Git-Hooks/pull/5
v1.0.2

  [Fixes]

  - (PR#5) Remove the need to use File::Slurp in Makefile.PL.