Update urbanairship.gemspec

Should prevent error message urbanairship-ae77dd9d9984 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  ["README.markdown"] are not files
Merge pull request #60 from MarkMurphy/patch-1

Update urbanairship.gemspec