Fix for missing dependency

`Perl6::Slurp` is not declared as test dependency in `Makefile.PL`.
So I got following error at `make test`.

```
% make test
PERL_DL_NONLAZY=1 /home/syohei/.plenv/versions/5.20.0/bin/perl5.20.0 "-MExtUtils::Command::MM" "-MTest::Harness" "-e" "undef *Test::Harness::Switches; test_harness(0, 'inc', 'blib/lib', 'blib/arch')" t/Dicom-DCMTK-DCMQRSCP-Config/*.t
t/Dicom-DCMTK-DCMQRSCP-Config/00-use.t ........... ok   
t/Dicom-DCMTK-DCMQRSCP-Config/01-version.t ....... ok   
t/Dicom-DCMTK-DCMQRSCP-Config/02-pod_coverage.t .. ok   
t/Dicom-DCMTK-DCMQRSCP-Config/03-pod.t ........... ok   
t/Dicom-DCMTK-DCMQRSCP-Config/04-new.t ........... ok   
t/Dicom-DCMTK-DCMQRSCP-Config/05-parse.t ......... Can't locate Perl6/Slurp.pm in @INC (you may need to install the Perl6::Slurp module) (@INC contains: /home/syohei/tmp/gomi/Dicom-DCMTK-DCMQRSCP-Config/inc /home/syohei/tmp/gomi/Dicom-DCMTK-DCMQRSCP-Config/blib/lib /home/syohei/tmp/gomi/Dicom-DCMTK-DCMQRSCP-Config/blib/arch /home/syohei/.plenv/versions/5.20.0/lib/perl5/site_perl/5.20.0/x86_64-linux-thread-multi /home/syohei/.plenv/versions/5.20.0/lib/perl5/site_perl/5.20.0 /home/syohei/.plenv/versions/5.20.0/lib/perl5/5.20.0/x86_64-linux-thread-multi /home/syohei/.plenv/versions/5.20.0/lib/perl5/5.20.0 .) at t/Dicom-DCMTK-DCMQRSCP-Config/05-parse.t line 8.
BEGIN failed--compilation aborted at t/Dicom-DCMTK-DCMQRSCP-Config/05-parse.t line 8.
t/Dicom-DCMTK-DCMQRSCP-Config/05-parse.t ......... Dubious, test returned 2 (wstat 512, 0x200)
No subtests run 
t/Dicom-DCMTK-DCMQRSCP-Config/06-serialize.t ..... ok   

Test Summary Report
-------------------
t/Dicom-DCMTK-DCMQRSCP-Config/05-parse.t       (Wstat: 512 Tests: 0 Failed: 0)
  Non-zero exit status: 2
  Parse errors: No plan found in TAP output
Files=7, Tests=18,  0 wallclock secs ( 0.05 usr  0.01 sys +  0.30 cusr  0.04 csys =  0.40 CPU)
Result: FAIL
Failed 1/7 test programs. 0/18 subtests failed.
make: *** [test_dynamic] Error 255
```