Bump pubspec to 3.1.0, we'll publish after I/O.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=156493658

Author: matanlurey

pubspec.yaml

@@ -1,5 +1,5 @@
 name: angular2
-version: 3.1.0-beta+1
+version: 3.1.0
 author: Dart Team <[email protected]>
 description: Framework for modern web applications
 homepage: https://webdev.dartlang.org/angular