Revert dependency slack-ruby-bot-server version to 0.12.0.

Author: crazyoptimist

slack-ruby-bot-server-events.gemspec

@@ -16,5 +16,5 @@ Gem::Specification.new do |spec|
   spec.files         = `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(spec)/}) }
   spec.require_paths = ['lib']
 
-  spec.add_dependency 'slack-ruby-bot-server', '>= 1.2.0'
+  spec.add_dependency 'slack-ruby-bot-server', '>= 0.12.0'
 end