Re-enable dev and stable SDK failures for Travis

Dev 1.19-dev.5 fixed it's issue with Ubuntu precise

pkgs/markdown/.travis.yml

diff --git a/pkgs/markdown/.travis.yml b/pkgs/markdown/.travis.yml
index d9ed98c..b115726 100644
--- a/pkgs/markdown/.travis.yml
+++ b/pkgs/markdown/.travis.yml
@@ -6,10 +6,3 @@
   - 1.18.1
   - 1.17.1
 script: tool/travis.sh
-matrix:
-  allow_failures:
-  # We need Travis to run Trusty for Dart 1.19+
-  # Dart issue here https://github.com/travis-ci/travis-ci/issues/6415
-  # General issue here: https://github.com/travis-ci/travis-ci/issues/5695
-  - dart: dev
-  - dart: stable