Instance type must be specified or the command ./databox.sh up fails

[ivyleavedtoadflax]

Failing to specify instance type with -i causes the following error on Ubuntu 16.04 and bash 4.3.48(1)-release (x86_64-pc-linux-gnu):

* aws_instance.box: Error launching source instance: InvalidParameterValue: Invalid value '' for InstanceType.
        status code: 400, request id: 47c7f83c-3ecd-4ebd-ae57-da1bbe71e612

Specifying -i t2.micro (for instance) solves the issue immediately. Would be good to test this on OSX. It doesn't appear to be related to #30 (reverting to /bin/sh still produces the same error).