add support for frank-based version end-point

The frank-based calabash-ios server has calabash-formatted version info at /calabash_version, not /version. This patch adds support for that by making the version info end-point configurable.

I added the concept of a `CALABASH_WITH_FRANK` env var to configure whether to use the new /calabash_version end-point. I'm not totally sure that's the best approach - feel free to ask for modifications if you have another idea.