switch pubspec to old style version constriant

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 9924432..69cf0f9 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 description: (renamed to "package:html")
 homepage: https://pub.dartlang.org/packages/html
 dependencies:
-  html: ^0.12.1
+  html: '>=0.12.1 <0.13.0'
 dev_dependencies:
   path: '>=0.9.0 <2.0.0'
   unittest: '>=0.10.0 <0.11.0'