Test on oldest supported SDK on Travis (#257)

diff --git a/.travis.yml b/.travis.yml
index cfc0906..79a9c47 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,7 @@
 
 dart:
   - dev
-  - stable
+  - 2.0.0
 
 dart_task:
   - test: --platform vm