Regenerate gemspec for version 0.18.0
Regenerate gemspec for version 0.18.0

nylas/nylas-ruby,aevy/nylas-ruby,aevy/nylas-ruby,stevenharman/nylas-ruby,hartator/nylas-ruby,nylas/nylas-ruby,aevy/nylas-ruby,hartator/nylas-ruby,hartator/nylas-ruby