Automate generation of rsa keys for integration tests

[davidalpert]

I was initially confused as to how to get the specs to pass cleanly before investigating #232.

These two small changes to the rake tasks should smooth that out:

• the :prepare task is now an explicit dependency of the :spec task so that it gets called even if someone doesn't notice the :test: task and invokes :spec independently; and
• the :prepare task now auto-invokes the jira:generate_public_cert task if the local keypair is missing, rather than failing with guidance.

[SimonMiaou]

@davidalpert Thank you for your contribution