chore(build): Upgrade to TypeScript@1.5.3

This change also makes us compliant with 1.6.0-dev compiler,
so we can do some experiments with apps that use 1.6 features
and compile against Angular.

We should probably add a travis build for 1.6 so we stay compatible
with both versions.

gulpfile.js

@@ -760,10 +760,10 @@ gulp.task('!pre.test.typings', ['docs/typings'], function() {
 });
 
 // -----------------
-// TODO: Use a version of TypeScript that matches what is used by DefinitelyTyped.
 gulp.task('test.typings', ['!pre.test.typings'], function() {
   return gulp.src(['typing_spec/*.ts', 'dist/docs/typings/angular2/angular2.d.ts'])
       .pipe(tsc({target: 'ES5', module: 'commonjs',
+                 experimentalDecorators: true,
                  noImplicitAny: true,
                  // Don't use the version of typescript that gulp-typescript depends on, we need 1.5
                  // see https://github.com/ivogabe/gulp-typescript#typescript-version