Corrected the parsing of the use_ami_role setting from sns.json.
This statement
  settings['sns']['use_ami_role'] || true
doesn't work for {"use_ami_role":false} because false || true == true.
As a result, useAmiRole would always return true no matter what the
setting was in sns.json.
Introduced a nil check instead.
Merge pull request #487 from ggrossman/master

Corrected the parsing of the use_ami_role setting from sns.json.