fix proguard for logback and dart version (#68)

Author: jaeopt

android/proguard-rules.txt

@@ -10,5 +10,6 @@
 # Optimizely
 -keep class com.optimizely.optimizely_flutter_sdk.** {*;}
 -keep class com.fasterxml.jackson.** {*;}
+# Logback
+-keep class ch.qos.** { *; }
 ##---------------End: proguard configuration ----------
-

pubspec.yaml

@@ -4,7 +4,7 @@ version: 2.0.0-beta
 homepage: https://github.com/optimizely/optimizely-flutter-sdk
 
 environment:
-  sdk: '>=2.16.2 <=3.1.2'
+  sdk: '>=2.16.2 <4.0.0'
   flutter: ">=2.5.0"
 
 dependencies: