Travis: do browser testing on Chrome (#106)

Hoping it's less flaky
diff --git a/.travis.yml b/.travis.yml
index ccc0864..6fb2f8d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,9 +6,7 @@
 
 dart_task:
   - test: --platform vm
-  # No parallelism on Firefox (-j 1)
-  # Causes flakiness – need to investigate
-  - test: --platform firefox -j 1
+  - test: --platform chrome
   - dartanalyzer
   - dartfmt