fix to resolve remotes branch correctly

I fixed the issue #76 .
`git rev-parse` failed when the remote branch not checkout to local.
For example, when accessing `development.pool.dev`, in builder server, `remotes/origin/development` branch exists correctly but `development` branch doesn't exist. So builder fail to execute `git rev-parse development` inside, then builder builds nothing.

To fix this bug, I added some recovery process to `git rev-parse` flow.
- If it's fail to find commit-ref specified by subdomain with executing `git rev-parse`, try to search the branch in 'remotes/*/*' branches again.
- `git rev-parse` was called in `hook.rb`, but it was hard to fix because written it's written in mruby, so I moved this function to `build_server`. Now, the proxy handler get commit-id by communicating with `build server` via `http://0.0.0.0:9000/resolve_git_commit/<commit-ref>`.
Add to underscore to repository-prefix as Docker does not permit
to set image name less than 4 characters
Merge remote-tracking branch 'origin/fix-rev-parse-fail' into fan
Merge pull request #77 from mookjp/fix-rev-parse-fail

Fix to resolve remotes branch correctly
Change to fetch everytime
Merge remote-tracking branch 'origin/change-to-fetch-everytime' into fan