Stop testing on Dartium (#15)

Fixes https://github.com/dart-lang/stream_channel/issues/14
diff --git a/.travis.yml b/.travis.yml
index d5c64e9..a951032 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,13 +1,12 @@
 language: dart
-sudo: false
+
 dart:
   - dev
   - stable
+
 dart_task:
   - test: --platform vm
   - test: --platform firefox -j 1
-  - test: --platform dartium
-    install_dartium: true
   - dartfmt
   - dartanalyzer