The required go version to build packer seems to be 1.2

Go 1.2 added comparison functions in http://golang.org/doc/go1.2#text_template.

Since `provisioner/puppet-masterless/provisioner.go` uses the `ne` operator, packer crashed(?) on go 1.1.2 with the following error

```autosetup git:(dev) $ packer validate packer/zookeeper.pack
Template validation failed. Errors are shown below.

Errors validating build 'amazon-instance'. 1 error(s) occurred:

* Error parsing execute_command: template: template:1: function "ne" not defined
```

Upgrading to 1.2 fixed the issue.

Therefore changing the docs to reflect this requirement. Also add a little function to the build script to verify go version.