Update version to 2.9.

src/compiler/core.ts

@@ -4,7 +4,7 @@
 namespace ts {
     // WARNING: The script `configureNightly.ts` uses a regexp to parse out these values.
     // If changing the text in this section, be sure to test `configureNightly` too.
-    export const versionMajorMinor = "2.8";
+    export const versionMajorMinor = "2.9";
     /** The version of the TypeScript compiler release */
     export const version = `${versionMajorMinor}.0-dev`;
 }