use chrome to test on Travis (#121)

diff --git a/.travis.yml b/.travis.yml
index 1bb80fe..c278db9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,7 +6,7 @@
 
 dart_task:
   - test: --platform vm
-  - test: --platform firefox -j 1
+  - test: --platform chrome -j 1
   - dartanalyzer: --fatal-infos --fatal-warnings .
   - dartfmt