Update pubspec.yaml (#40)

Don't depend on dev releases unless we really, really have to

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index e79f947..81c2baa 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,7 +5,7 @@
 homepage: https://github.com/dart-lang/resource
 
 environment:
-  sdk: '>=2.0.0-dev.61.0 <3.0.0'
+  sdk: '>=2.0.0 <3.0.0'
 
 dependencies:
   typed_data: "^1.0.0"