Update & improve xcode-install to work with fastlane.ci

[KrauseFx]

This is needed for a better integration with https://fastlane.ci

• Sorted #seedlist

• seedlist is public now

• Xcode object has installed attribute that's not cached in-between runs
• Improve #exist? method to work with Gem::Version as well as plain strings (e.g. for "4.3 for Lion")
• Add and use new #find_xcode_version method with the above mentioned improvements

Related to https://github.com/fastlane/ci/pull/819

[KrauseFx]

Thanks for the comments @segiddins, will update the PR 👍

[KrauseFx]

@segiddins updated ✅

[KrauseFx]

Unfortunately https://developer.apple.com/download/more/ doesn't expose the exact build number of each Xcode release, so we can't use that to identify and download an Xcode release, will try to come up with something better