docs(quickstart): update Dart QS to beta.0

closes #534

public/docs/_examples/quickstart/dart/ex1/pubspec.yaml

@@ -2,7 +2,7 @@
 name: angular2-getting-started
 version: 0.0.1
 dependencies:
-  angular2: 2.0.0-alpha.45
+  angular2: 2.0.0-beta.0
   browser: ^0.10.0
 transformers:
 - angular2:

public/docs/_examples/quickstart/dart/ex2/pubspec.yaml

@@ -2,7 +2,7 @@
 name: angular2-getting-started
 version: 0.0.1
 dependencies:
-  angular2: 2.0.0-alpha.45
+  angular2: 2.0.0-beta.0
   browser: ^0.10.0
   dart_to_js_script_rewriter: ^0.1.0
 transformers:

public/docs/dart/latest/quickstart.jade

@@ -40,8 +40,8 @@ p.
     In <code>pubspec.yaml</code>,
     specify the angular2 and browser packages as dependencies,
     as well as the angular2 transformer.
-    Angular 2 is changing rapidly, so provide an exact version:
-    <b>2.0.0-alpha.45</b>.
+    Angular 2 is still changing, so provide an exact version:
+    <b>2.0.0-beta.0</b>.
 
   +makeExample('quickstart/dart/ex1/pubspec.yaml', null, 'pubspec.yaml')