Update Travis-CI config to use dev channel (#24)

.travis.yml

@@ -1,19 +1,12 @@
 language: dart
 
 dart:
-  - preview/raw/2.10.0-0.2-dev
+  - dev
 
 dart_task:
   - test: --platform vm,chrome
-
-matrix:
-  include:
-    # Only validate formatting using the preview/raw/2.10.0-0.2-dev release
-    - dart: preview/raw/2.10.0-0.2-dev
-      dart_task: dartfmt
-    - dart: preview/raw/2.10.0-0.2-dev
-      dart_task:
-        dartanalyzer: --fatal-warnings --fatal-hints .
+  - dartfmt
+  - dartanalyzer: --fatal-warnings --fatal-hints .
 
 # Only building master means that we don't run two builds for each pull request.
 branches: