Fix travis by requiring 1.22.0

Fixes https://github.com/dart-lang/observable/issues/34
diff --git a/.travis.yml b/.travis.yml
index e746676..fbe169b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@
 dart:
   - stable
   - dev
-  - 1.21.0
+  - 1.22.0
 env:
   - TEST_PLATFORM=vm
   - TEST_PLATFORM=firefox