Specify the required boto version in the README.

It needs to be clear that we need a newer version of boto than is included in Wheezy's python-boto package.

Otherwise:
```
Registering the image as an AMI
register_image() got an unexpected keyword argument 'virtualization_type'
Traceback (most recent call last):
  File "~/bootstrap-vz/base/main.py", line 38, in run
    tasklist.run(info=bootstrap_info, dry_run=args.dry_run)
  File "~/bootstrap-vz/base/tasklist.py", line 29, in run
    task.run(info)
  File "~/bootstrap-vz/providers/ec2/tasks/ami.py", line 170, in run
    info.image = info.connection.register_image(**registration_params)
TypeError: register_image() got an unexpected keyword argument 'virtualization_type'
Rolling back
Removing the bundle files
Deleting the volume
Deleting workspace
Successfully completed rollback
root@domU-AA-BB-CC-DD-EE-FF:~/bootstrap-vz# apt-cache policy python-boto
python-boto:
  Installed: 2.3.0-1
  Candidate: 2.3.0-1
  Version table:
 *** 2.3.0-1 0
        500 http://http.debian.net/debian/ wheezy/main amd64 Packages
        100 /var/lib/dpkg/status
```

Required boto commit:
https://github.com/boto/boto/commit/e14f71da995672de43599ba0e10f8e7ed2d4d471