Only test on the dev SDK (#111)

diff --git a/.travis.yml b/.travis.yml
index 07d647d..6d64d6a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,6 @@
 language: dart
 
 dart:
-  - 2.2.0
   - dev
 
 dart_task:
@@ -12,9 +11,6 @@
     - dart: dev
       dart_task:
         dartanalyzer: --fatal-infos --fatal-warnings .
-    - dart: 2.2.0
-      dart_task:
-        dartanalyzer: --fatal-warnings .
     - dart: dev
       dart_task: dartfmt