Travis: add `xvfb: false` (dart-lang/markdown#195)

diff --git a/pkgs/markdown/.travis.yml b/pkgs/markdown/.travis.yml
index 8e02a6b..8bd12ad 100644
--- a/pkgs/markdown/.travis.yml
+++ b/pkgs/markdown/.travis.yml
@@ -7,7 +7,8 @@
   - 1.22.1
   - 1.21.1
 dart_task:
-  - test: -p vm
+  - test: --platform vm
+    xvfb: false
   - dartfmt
   - dartanalyzer