Regenerate gemspec for version 0.15.8
Regenerate gemspec for version 0.15.8

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