Don't test under 1.21.1 (#33)

Context: https://github.com/dart-lang/pub_semver/commit/7b2549d1f57d9311f5525c761ce7627bdb6e6d27#r29835939
diff --git a/.travis.yml b/.travis.yml
index 6788dee..c654072 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,6 @@
 dart:
   - dev
   - stable
-  - 1.21.1
 
 dart_task:
   - test