Downgraded flutter version to 1.17.0 (#228)

diff --git a/packages/flutter_markdown/pubspec.yaml b/packages/flutter_markdown/pubspec.yaml
index 075fdac..66a984b 100644
--- a/packages/flutter_markdown/pubspec.yaml
+++ b/packages/flutter_markdown/pubspec.yaml
@@ -18,7 +18,7 @@
 
 environment:
   sdk: '>=2.5.0 <3.0.0'
-  flutter: '>=1.17.1 <2.0.0'
+  flutter: '>=1.17.0 <2.0.0'
 
 flutter:
   assets: