Bump the version to 0.6.7+2. (#67)

diff --git a/pubspec.yaml b/pubspec.yaml
index 7fdad5e..c331e58 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: shelf
-version: 0.6.7+1
+version: 0.6.7+2
 author: Dart Team <misc@dartlang.org>
 description: Web Server Middleware for Dart
 homepage: https://github.com/dart-lang/shelf