Update sigdump to 0.2.5

[daipom]

Which issue(s) this PR fixes:
None.

What this PR does / why we need it:
In 0.2.5, the bug of wrong value of hash pairs and array elements fixed.

That version will be installed without this fix, but I think we should explicitly update the version.
(Is it correct?)

We can confirm it as follows on non-Windows. (On Windows, we can use fluent-ctl.)

$ sudo kill -CONT {pid}
$ less /tmp/sigdump-{pid}.log

Before: Too small Array and Hash sizes

...
   Array 61 elements
    Hash 8 pairs

After: Correct size

...
   Array 41,555 elements
    Hash 15,752 pairs

Docs Changes:
No need.

Release Note:
Fix wrong Object Array and Hash sizes of SIGDUMP.

ETC:
Should we merge this into v1.16 too?

[ashie]

Should we merge this into v1.16 too?

Not always needed.
Even if we don't backport it, the latest version will be installed.
And we'll bundle latest version of sigdump explicitly for td-agent (fluent-package).

[daipom]

Thanks for your review!