0.6.1: Fix terraform validate test (#11)

* Adds prerequisites to README

* Fixes validate.bats issue with "region" error for modules. 

This is the same issue as 
https://github.com/cloudposse/build-harness/pull/208. Without a default 
region, `terraform validate` fails when running against independent 
modules as they do not have a provider/region supplied.

* Fixes validate.bats unsetting the wrong environment var

I believe this was just a mistake but I may be wrong. setup() sets 
TF_CLI_ARGS_init, but teardown unsets TF_CLI_ARGS_plan. That seemed like 
a small typo to me, so I figured I would include this.