Add deploy-version subtask

From the section I added to the README.md:

#### Rolling Version Deploy

A normal deployment will incur some downtime, as both the environment
settings update and the version deployment will stop the Tomcat server
on all EC2 instances at once. There are several work-arounds for this
(see [Deploying Versions with Zero Downtime][7],
[Demystified – Zero Downtime with AWS Elastic Beanstalk][8], and my
preferred Blue-Green approach from [ThoughtWorks][9]), but all of them
have drawbacks compared to Elastic Beanstalk's new (as of October 2014)
[Rolling Version Deploy feature][10].

If you are only deploying a new version and **not** changing any
environment settings (i.e. Software Configuration > Environment Properties
in the AWS Elastic Beanstalk console for your environment), you can
use the `deploy-version` command:

    $ lein beanstalk deploy-version development

You can also specify a custom WAR as above.

`deploy-version` will verify that you have no differences between the
Elastic Beanstalk environment's settings and the environment's `:env`
map in `:aws :beanstalk :environments` in your `project.clj`. If there
are any differences, the deployment will abort through an exception
and you will need to proceed with a normal deployment in order to update
the environment settings automatically.

[7]: http://docs.aws.amazon.com/elasticbeanstalk/latest/dg/using-features.CNAMESwap.html
[8]: http://www.hudku.com/blog/demystified-zero-downtime-with-amazon/
[9]: http://www.thoughtworks.com/insights/blog/implementing-blue-green-deployments-aws
[10]: http://docs.aws.amazon.com/elasticbeanstalk/latest/dg/using-features.rolling-version-deploy.html