code: def get_installed_version(vcs):
      
      version_path = _get_version_path(vcs)
      if not os.path.exists(version_path):
          raise VersionNotInstalledError
      with open(version_path, 'r') as f:
          return f.read().strip()
docs: Get the installed version for this project.
  
      Args:
          vcs (easyci.vcs.base.Vcs)
  
      Returns:
          str - version number
  
      Raises:
          VersionNotInstalledError