Bump prebuilt-image version in demo dockerfile

[pingsutw]

Update prebuild image version to v0.55 due to v0.54 can't find ppa:jonathonf/python-3.6
This ppa has been removed from public. https://launchpad.net/~jonathonf/+archive/ubuntu/python-3.6

error message :

Ign:22 http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu xenial/main all Packages    
Err:25 http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu xenial/main amd64 Packages  
  403  Forbidden [IP: 91.189.95.83 80]
Fetched 4461 kB in 11s (400 kB/s)                                                       
Reading package lists... Done
W: The repository 'http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu/dists/xenial/main/binary-amd64/Packages  403  Forbidden [IP: 91.189.95.83 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.

[pingsutw]

@tqchen, Thanks for the review