Travis testing: dev, stable, and oldest supported (dart-lang/yaml#33)

* Travis testing: dev, stable, and oldest supported
* Require SDK >= 1.23.0 - range we can correctly test on travis

pkgs/yaml/.travis.yml

diff --git a/pkgs/yaml/.travis.yml b/pkgs/yaml/.travis.yml
index 15f2fd2..d0fdea5 100644
--- a/pkgs/yaml/.travis.yml
+++ b/pkgs/yaml/.travis.yml
@@ -1,12 +1,9 @@
 language: dart
 sudo: false
 dart:
-  - stable
   - dev
-  - 1.22.1
-  - 1.21.1
-  - 1.20.1
-  - 1.19.1
+  - stable
+  - 1.23.0
 dart_task:
   - test: -p vm
     xvfb: false

pkgs/yaml/pubspec.yaml

diff --git a/pkgs/yaml/pubspec.yaml b/pkgs/yaml/pubspec.yaml
index 2b65030..8dd0034 100644
--- a/pkgs/yaml/pubspec.yaml
+++ b/pkgs/yaml/pubspec.yaml
@@ -1,8 +1,10 @@
 name: yaml
-version: 2.1.12
+version: 2.1.13-dev
 author: "Dart Team <misc@dartlang.org>"
 homepage: https://github.com/dart-lang/yaml
 description: A parser for YAML.
+environment:
+  sdk: '>=1.23.0 <2.0.0-dev.infinity'
 dependencies:
   charcode: "^1.1.0"
   collection: ">=1.1.0 <2.0.0"
@@ -11,5 +13,3 @@
 dev_dependencies:
   path: ">=1.2.0 <2.0.0"
   test: ">=0.12.0 <0.13.0"
-environment:
-  sdk: '>=1.12.0 <2.0.0-dev.infinity'