prep for publishing 1.5.3 (#133)

prep for publishing 1.5.3
diff --git a/pubspec.yaml b/pubspec.yaml
index 9d4301e..3d71214 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: args
-version: 1.5.3-dev
+version: 1.5.3
 homepage: https://github.com/dart-lang/args
 description: >-
  Library for defining parsers for parsing raw command-line arguments into a set