Remove old VM flag from Travis config. (#706)

diff --git a/.travis.yml b/.travis.yml
index caf1bc3..8b6048d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,5 @@
 language: dart
 sudo: false
-env:
-  - DART_VM_OPTIONS=--preview-dart-2
 dart:
   - dev
 dart_task: