Declare support for the latest fake_async (#59)

Closes #49
diff --git a/pubspec.yaml b/pubspec.yaml
index e8dafaa..3183b45 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -8,7 +8,7 @@
 dependencies:
   collection: "^1.5.0"
 dev_dependencies:
-  fake_async: "^0.1.2"
+  fake_async: ">=0.1.2 <2.0.0"
   stack_trace: "^1.0.0"
   test: "^0.12.0"
   # For building and testing with DDC