Bump the pubspec version to 1.5.0.

R=cbracken@google.com

Review URL: https://codereview.chromium.org//2041813003 .
diff --git a/pubspec.yaml b/pubspec.yaml
index 1c1df31..3428dac 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: stream_channel
-version: 1.4.0
+version: 1.5.0
 description: An abstraction for two-way communication channels.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/stream_channel