Do not use awk to read VERSION file
Support GT_AWS_ZONE env variable

Setting GT_AWS_ZONE to one of the availability zone (not the region,
i.e. 'us-west-2a' not 'us-west-2') will set the default zone for
init/terminate commands.

-z option in both commands is now opzional.
Updated gemspec
Required gaptool-api 0.4.0
Removed discontinued features
Bumped to 0.6.0