[swift-corelibs-dev] Update on toolchain and XCTest requirements for Foundation

[Philippe Hausler]

Just as a note to those who may be working on Foundation; we have a few version requirements that are required to build Foundation on both Ubuntu and Mac OS X.

XCTest must include the commit https://github.com/apple/swift-corelibs-xctest/commit/e5cf2f401867ab90b1b307b4b825f2e7fafddf51 which introduces the throws attribute on the array of test cases.
The swift toolchain must recent as of 2016-01-15 to include the new keyword replacement for typealias; associatedtype.

Hope that saves others time hunting down the right versions to build.