Update README.version

external/llvm is no longer rolling from upstream. Update README.version
to better reflect that.

Change-Id: I7172dd60ce6a46114812494a64fdd15746050830

README.version

 URL: http://llvm.org/git/llvm.git
-Version: Rolling from upstream + cherry-picks
+Version: r275480 + cherry-picks
 BugComponent: 117395
 Owners: srhines, android-llvm