Travis: merge test tasks, use Chrome instead of firefox (#103)

Chrome seems a bit less flaky
diff --git a/.travis.yml b/.travis.yml
index df7d186..07d647d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,15 +5,16 @@
   - dev
 
 dart_task:
-  - test: --platform vm
-  - test: --platform firefox -j 1
+  - test: --platform vm,chrome
 
 matrix:
   include:
     - dart: dev
-      dartanalyzer: --fatal-infos --fatal-warnings .
+      dart_task:
+        dartanalyzer: --fatal-infos --fatal-warnings .
     - dart: 2.2.0
-      dartanalyzer: --fatal-warnings .
+      dart_task:
+        dartanalyzer: --fatal-warnings .
     - dart: dev
       dart_task: dartfmt