xiki.gemspec needs to be updated

...d, reflecting change in 86d3ed132407ec3852c01b1a6d5007263f608817

Changed 
  s.extra_rdoc_files = [
    "LICENSE",
    "README.markdown"
  ]

to
  s.extra_rdoc_files = [
    "LICENSE",
    "README.md"
  ]

Reflects changes made in commit 86d3ed132407ec3852c01b1a6d5007263f608817.

Otherwise, xiki.gemspec notes error of not having a valid gemspec.