Minor version bump v1.19.0

Updates Remote API to 1.16
Adds exit code to Docker::Exec#start!
Calls `close_write` on IO if necessary after STDIN attach ends

Author: tlunter

lib/docker/version.rb

@@ -1,6 +1,6 @@
 module Docker
   # The version of the docker-api gem.
-  VERSION = '1.18.0'
+  VERSION = '1.19.0'
 
   # The version of the compatible Docker remote API.
   API_VERSION = '1.16'