Explicitly test on earliest supported stable SDK (dart-lang/stream_transform#74)

This way when a new stable is released it won't stop the tests on a
supported SDK version.

pkgs/stream_transform/.travis.yml

diff --git a/pkgs/stream_transform/.travis.yml b/pkgs/stream_transform/.travis.yml
index e6d16f6..5cdff4c 100644
--- a/pkgs/stream_transform/.travis.yml
+++ b/pkgs/stream_transform/.travis.yml
@@ -3,7 +3,7 @@
   only: [master]
 dart:
   - dev
-  - stable
+  - 2.2.0
 cache:
   directories:
     - $HOME/.pub-cache