Stop using File::Slurp

it's not a core module, and more importantly, the error
message sucks when it's missing (because the TAP harness
swallows it up by default).

tools/jvm/eval-client.pl

@@ -3,9 +3,17 @@
 use warnings;
 
 use IO::Socket::INET;
-use File::Slurp 'read_file';
 use Encode ();
 
+sub read_file {
+    my $filename = shift;
+    open my $IN, '<', $filename or die "Error while reading '$filename': $!";
+    local $/;
+    my $contents = <$IN>;
+    close $IN;
+    return $contents;
+}
+
 if (@ARGV < 2) {
     print STDERR <<USAGE ;
 Usage: tools/jvm/eval-client.pl TOKEN_FILE exit