bump silk
bump silk

[#144881385]

Submodule src/code.cloudfoundry.org/silk 72dba8b..668cc9e:
  > Add logic for acquiring new lease if ip not in overlay network and there are no containers running

Signed-off-by: Joachim Valdez <f4115f6bcd6858c277581a1d9da83b9914dce2c3@us.ibm.com>

cloudfoundry-incubator/cf-networking-release,cloudfoundry-incubator/cf-networking-release,cloudfoundry-incubator/cf-networking-release,cloudfoundry-incubator/cf-networking-release