Updated SDK version to match that used by Travis CI

BUG=
R=zra@google.com

Review-Url: https://codereview.chromium.org/2836873004 .
diff --git a/pubspec.yaml b/pubspec.yaml
index e4eba58..7cb1f46 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,6 +4,6 @@
 description: Synchronous HTTP client for Dart.
 homepage: https://github.com/dart-lang/sync_http
 environment:
-  sdk: '>=1.24.0-dev.1.0 <2.0.0'
+  sdk: '>=1.24.0-dev.0.0 <2.0.0'
 dev_dependencies:
   test: ">=0.12.0 <0.13.0"