Update ruby-raindrops to 0.15.0.

=== raindrops 0.15.0 - non-glibc compat fix on Linux / 2015-07-22 00:30 UTC

  Thanks to Doug Forster for sending us the report.
  No other fixes

  * check for the existence of linux/tcp.h

=== raindrops 0.14.0 - misc doc updates / 2015-06-25 21:50 UTC

  Eric Wong (7):
        linux_inet_diag: annotate memory freeing on diag errors
        README: trim intro and update license
        modernize packaging and documentation
        move mailing list to raindrops-public@bogomips.org
        linux_inet_diag: clarify *fprintf usage without GVL
        TODO: add item for unix_diag and udp_diag
        linux_inet_diag: fix Wshorten-64-to-32 warnings

  Hleb Valoshka (1):
        Add setup and teardown for ipv6 tests

(taca)