swift234

Bump the ABI version for Swift 3.0.

This was previously bumped to move from Swift 1.2 to 2.0.  Time for 3.0.

Note, unlike last time (https://github.com/apple/swift/commit/136965d9f8eb1780f3a1f830bdb3fd8b9cd46d38), the
CMakeLists.txt change already says 3.0 so that won't get changed here.