Try this

Author: bobtfish

synapse.gemspec

@@ -17,7 +17,7 @@ Gem::Specification.new do |gem|
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})
 
   gem.add_runtime_dependency "aws-sdk", "~> 1.39"
-  gem.add_runtime_dependency "docker-api", "~> 1.7.2"
+  gem.add_runtime_dependency "docker-api", "~> 1.7.2", :require => 'docker'
   gem.add_runtime_dependency "zk", "~> 1.9.4"
   gem.add_runtime_dependency "etcd", "~> 0.2.4"