Use server sent event(SSE) instead of websocket

Good bye extra port. #38
This feature replace websocket with [server sent
event](http://www.w3schools.com/html/html5_serversentevents.asp).

`Builder::BuildHandler` is a server responses the build logs by `GET`
request as Server sent event stream.
`Builder::BuildHandler` listens on the port `9001`, if you access the
`http://localhost:9001/build/master` by `GET` request, the build server
starts to build  the `master` container  and puts build log streams.)]
spec: fix build server on specific branch
Merge pull request #109 from mookjp/use-server-sent-event

Use server sent event