Bump version to 0.5.0

BUG=
R=sgjesse@google.com

Review URL: https://chromiumcodereview.appspot.com//1379453002.
diff --git a/pubspec.yaml b/pubspec.yaml
index 569095d..f1db72a 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,12 @@
 name: protoc_plugin
-version: 0.4.2
+version: 0.5.0
 author: Dart Team <misc@dartlang.org>
 description: Protoc compiler plugin to generate Dart code
 homepage: https://github.com/dart-lang/dart-protoc-plugin
 environment:
   sdk: '>=1.0.0 <2.0.0'
 dependencies:
-  protobuf: '>= 0.4.2 <0.5.0'
+  protobuf: '>= 0.5.0 <0.6.0'
   path: '>=1.0.0 <2.0.0'
 dev_dependencies:
   browser: any