bump garden
bump garden

[#156040535]

Submodule src/code.cloudfoundry.org/garden 61ba66a..b6b9880:
  > Use golang's standard sync map

Signed-off-by: Julia Nedialkova <7434cf6db6c17ad5b6ef614abe065a9cda1d30c1@hotmail.com>

cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release