only test dev and stable (dart-lang/markdown#200)

pkgs/markdown/.travis.yml

diff --git a/pkgs/markdown/.travis.yml b/pkgs/markdown/.travis.yml
index 3b057ea..3456f5b 100644
--- a/pkgs/markdown/.travis.yml
+++ b/pkgs/markdown/.travis.yml
@@ -3,9 +3,6 @@
 dart:
   - stable
   - dev
-  - 1.23.0
-  - 1.22.1
-  - 1.21.1
 dart_task:
   # Using the expanded reporter to work-around
   # https://github.com/dart-lang/markdown/issues/191