add sdk requirements to pubspec

pubspec.yaml

@@ -4,6 +4,9 @@ version: 1.3.0
 author: Posse Productions LLC <apps@goposse.com>
 homepage: https://github.com/goposse/fluro
 
+environment:
+  sdk: ">=1.23.0 <2.0.0"
+
 dependencies:
   flutter:
     sdk: flutter