Expose MyriaX version in API

[senderista]

This was implemented long ago in MyriaX: uwescience/myria#498

$ curl localhost:8753/server                                                                                                                                                              
{"commit":"f5694a532f1c826f50e678127d32a44d317c55c7","branch":"reef"}

It would be useful to expose this REST API through the myria-python API as well.