Fix to handle subdomain as git-commit-id inside pool

- Fix hook.rb and build_server not to create another container
  if the same commit-id is given so that it targets the same container
  if it gets branch name or tag.
- Fix Dockerfile to test builder gem while building the container
- Modified application's repository path is on fixed path; `app_repo`
- Refactor hook.rb
Merge pull request #61 from mookjp/issue-33_see-actual-commit-id

#33 Fix to see actual git-commit-id if branch or tag name was given