Test oldest SDK version on Travis (#76)

diff --git a/.travis.yml b/.travis.yml
index 2ea8538..dd151e8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,6 +2,7 @@
 
 dart:
   - dev
+  - 2.0.0
 
 dart_task:
   - test: --platform vm