Bump min required SDK (#28)

Allows reliable testing
diff --git a/.travis.yml b/.travis.yml
index dfee504..e758639 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@
 dart:
   - stable
   - dev
-  - 1.20.1
+  - 1.23.0
 dart_task:
   - test: -p vm
     xvfb: false
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 15e1f6d..270e306 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,6 @@
-# Changelog
+## 2.1.4
+
+- Require at least Dart 1.23.0.
 
 ## 2.1.3
 - Fix bug in `readAsBytes` which returned twice as much data as expected.
diff --git a/pubspec.yaml b/pubspec.yaml
index 12866dc..1b2ea68 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,11 +1,11 @@
 name: resource
-version: 2.1.3
+version: 2.1.4-dev
 description: Reading resource data from (package and other) files.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/resource
 
 environment:
-  sdk: '>=1.20.0 <2.0.0'
+  sdk: '>=1.23.0 <2.0.0'
 
 dependencies:
   typed_data: "^1.0.0"