Switch Travis-CI to use Chrome over Firefox (#436)

Less flaky
diff --git a/.travis.yml b/.travis.yml
index f0dd98d..6503749 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,10 +5,7 @@
   - 2.4.0
 
 dart_task:
-  - test: --platform vm
-  # No parallelism on Firefox (-j 1)
-  # Causes flakiness – need to investigate
-  - test: --platform firefox -j 1
+  - test: --platform vm,chrome
   - dartanalyzer: --fatal-infos --fatal-warnings .
 
 matrix: