Don't crash if the connection has an error
set encrypted flag to None for root device

"AWS API doesn't support any value for this flag for the root device of
a new instance"

see: boto#2587

closes gh-455
move try/except from is_up to is_ssh_up
Merge branch 'fix-conn-reset-by-peer' into develop

closes #465
remove t2 types from placement group types list
bump version to 0.95.6
Changelog: update for 0.95.6 release
Merge branch 'hotfix-0.95.6' into develop
minor experiment/mnist.run tweaks
Merge github.com:jtriley/StarCluster into develop

* github.com:jtriley/StarCluster:
  Changelog: update for 0.95.6 release
  bump version to 0.95.6
  remove t2 types from placement group types list
  move try/except from is_up to is_ssh_up
  set encrypted flag to None for root device
  Don't crash if the connection has an error