remove dependency on File::Slurp

remove dependency on File::Slurp

The current version of File::Slurp has a "use 5.6.2;" statement. This
throws a warning on perl 5.10.0 which breaks some of our tests. The
statement should be "use 5.006002;".

My attempts to contact the author, Uri Guttman were not successful.

As we have our own convenience method 'slurp_file' in ::Utils now, there
is little need to depend on File::Slurp anymore anyway.

For these reasons the dependency has been removed.