Support more flexible version selection #452

Since Fastlane moved to using xcodes for version selection and install, we've lost the ability to be more flexible in the versions we select. For instance, attempting to select version 26.0 when 26.0.1 is installed will fail. I'd like to suggest one of three improvements:

1. Automatically allow any patch version when selecting an Xcode minor version. e.g. 26.0 would match 26.0.2 but not 26.1.
2. Like 1, but require a version level match. e.g. 26.0.0 would match the latest patch version, but 26.0 would still require a specific version.
3. Match the previous Ruby tool behavior and allow the use of the ~> operator with a provided version. e.g. ~> 26 would match the latest major version available, ~> 26.0 would match the latest minor version available, and ~> 26.0.0 would match the latest patch version of 26.0.

This would allow CI to remain within a supported version range without an explicit update.